PROSPECTING FOR EMERALDS

AFRICAN CLUES TO THE GREAT EMERALD MYSTERY

By AllanTaylor  FGA

First published in the US Lapidary Journal in June, 1976

Reprinted here in 2019 to provide Internet access to valuable information on emerald occurrences and the prospecting for them.   When I was there the country was called Rhodesia with Ian Smith as Prime Minister. In 1980,  Mr Robert Mugabe became PM and things changed considerably.  The county became Zimbabwe and all English place names were removed, e.g., Salisbury, the capital, became Harare.   I have not made these place name changes in the text since it would only confuse matters.  The original text is presented here, and I may be able to find and reproduce some maps and fotos, at a later date.

RHODESIA,  HERE I COME

The sky was blue and cloudless. The lightly forested landscape appeared friendly and interesting. Our little Toyota shuddered violently from the corrugations on the dusty dirt road.  Granite country, of course, you could tell this even with your eyes shut. Next we passed over a smooth section which was across a serpentine belt, and then turned into the driveway of the Girdlestone Ranch.  It was a happy day for me.  For we were about to visit an emerald and alexandrite prospect.  Yes, this was emerald country alright; on the central ancient granitic shield region of Rhodesia.  Within a stretch of a few hundred miles, there are a dozen or more emerald prospects, small mines and one large mine, Rio Tinto’s famous Sandawana deposit which is located further to the southwest, but unfortunately closed to visitors.

The formation of emerald crystals, be they synthetic or natural ones, is the happy result of various processes that are a great mystery, and only understood by scientists in a general sense.  There is a lot to be learnt from a careful study of the natural occurrences of emeralds; knowledge that can be applied both for prospecting for new deposits, or guidance in developing and improving synthetic methods of growth.

Considering the World as a whole, the formation of emerald is a rare happening in the geological past. There are very few emerald deposits compared to the number of known beryl deposits.  Emerald is the name given to the bright green variety of the mineral beryl, which is a beryllium aluminium silicate.  Beryl is the chief ore mineral for the valuable metal beryllium, used to make exotic alloys.  It is a comparatively common mineral, but has a very restrictive mode of occurrence, for it is nearly always found in or closely associated with granite pegmatites.  The great rarity of emerald compared to beryl, which is mined by the thousands of tons around the world, was first explained satisfactorily by the famous Norwegian geochemist V. M. Goldschmidt.  He pointed out that the two essential elements for the formation of emerald viz., beryllium and the green coloring element chromium (usually, but sometimes vanadium or both), do not become concentrated in the same rock types.  In fact, their abundances are exact opposites due to their differing physicochemical properties.

Beryllium is such a small ion that it is not able to become incorporated into the crystal structure of the common rock forming minerals such as feldspars, pyroxenes and micas when they crystallize from a cooling magma.  Thus great masses of igneous rocks like gabbro, diorite and even granite contain only insignificant amounts of beryllium.  Beryllium is a sly element.  A misfit really.  Its concentration builds up in the water-rich dregs of a cooling granite intrusion from which veins of pegmatite may sprout out into the country rock.  It is these wonderful pegmatite veins which are so often the source of many gemstones, together with large crystals of quartz, feldspar and mica.  Sometimes enormous crystals of beryl weighing several tons are found, but most frequently they are much smaller, measuring 1 to 10 cm across the prism.  Normally these crystals are opaque and of pale color, off-white to dingy green, sometimes yellowish or bluish, certainly not having the vivid green fire of emerald.  And there arises another mystery, why do most crystals grow opaque, full of flaws and tiny fluid inclusions? Why are transparent, faceting grade crystals so rare, be they beryl, quartz or topaz or whatever?

Those of you familiar with the New England beryl-containing pegmatites will understand the normal state of affairs.  Beryls that crystallize within the pegmatite are not usually strongly colored because these residual solutions are practically devoid of any pigment elements, particularly chromium and vanadium.  Always some iron is available.  This is the coloring element most frequently found in pegmatitic beryls (0.1 to 2.0%) and is responsible for the usual pale green, yellow and blue shades.

Goldschmidt explained that it was only when a beryllium-bearing pegmatite intruded into rock types having a high abundance of chromium or vanadium that there would be any chance of the two essential elements of getting together in appropriate amounts for emerald to form. These rock types would be the mafic igneous rocks and their metamorphosed equivalents e.g., commonly dunite, peridotite, serpentine, tremolite and mica schists etc.  Such a geological happening is not very common on a world-wide basis, hence the rarity of emerald compared to common beryl.  One does not have to search very far in the literature to find examples of what may be exceptions to the “rule”.  At the famous Chivor and Muzo Mines in Columbia, emeralds are found in calcite-quartz veins traversing black carbonaceous limestone and shale.  Even so, Goldscmidst’s geochemical hypothesis that the beryllium must be brought to the chrome because these two elements are not naturally transported together, is still appropriate for these atypical deposits.  A useful generalization is that in 90% of occurrences, emeralds are found in mica or amphibole schists in close asscociation with granite pegmatite veins.  The Rhodesian emerald occurrences are very typical, as noted by previously by geologists of the Geological Survey in Salisbury.

The Novello claims that we were about to visit are located about 13 miles north-east of Fort Victoria.  They were discovered by Mrs C. Girdlestone in September 1960.  It should be noted that all the pegmatite veins and mineralized areas are on registered  mining claims and are not generally open for collecting.

After getting permission at the Girdlestone homestead, we headed our little car down the dirt track that disappeared into the scrub.  Our guide was Cyril Gurr, the surveyor from the Mines Department.  He had mapped all the workings some years ago and knew all the local prospectors and landowners.  Don, a retired mining engineer from Colorado, made up the third member of party.  A bouncy two mile trip brought us to the end of the trail on the bank of the Chipopoteke River.  A newly constructed suspension bridge was strung across the murky waters, which looked very promising for bream.  A massive quartz-feldspar pegmatite outcropped on the foundations of the bridge.  We proceeded on foot for about half a mile across flat scrub covered land.  The geology of the area seemed most complex.  Serpentine, schist, then banded ironstone, here and there massive quartz blows and pegmatite veins were outcropping.  Every now and then there was a scratching, a trench or prospect pit.  Finally, we arrived at the main open cut which was being worked for …..no, not emeralds, but the even rarer beryllium mineral and gemstone, alexandrite.

What has been said about emerald formation is also true of alexandrite, which is a chromium colored variety of the mineral chrysoberyl, in composition a beryllium aluminum oxide.  Unlike emerald, it contains no silica, so a third prerequisite is a silica deficient environment.  Thus alexandrite is frequently associated with corundum (but the reverse is not true of course). When beryllium-bearing solutions have migrated far enough away from a pegmatite vein to become silica deficient then perhaps alexandrite will crystallize.  This seemed to be the case here at what is called the Novello Alexandrite Prospect.  No pegmatite veins where apparent in the near vicinity.  It is not surprising that the number of alexandrite deposits in the world can be counted on the fingers of one hand.

The mine manager, another Cyril, and four African workmen, were on the site.  We were shown over the workings and the details were explained.  The country rock was serpentine, somewhat altered and criss-crossed with a veining of a white mineral that I could only guess the identity of. The alexandrite was confined to narrow (10 to 100 cm) seams or lenses of  phlogopite mica. The phlogopite rock was well compacted and difficult to break up as there was no schistosity, nor were there planes of weakness, hence it was often a problem to remove specimens of alexandrite without damaging them.  The #1 mica seam was worked to a depth of about 4 meters until the exaction became unsafe.  The serpentine when wet is apparently too soft for timber support.  Next the overburden was removed further back revealing in the process two more parallel mica seams and a couple of lenses.  These new seams were currently being exploited and the mica rock stockpiled nearby for later treatment and recovery of alexandrite.  More easily recovered were clusters of alexandrite crystals localized along smooth slickensided surfaces in the mica vein, particularly along the contact with the serpentine. 

The alexandrite occurred as characteristic cyclical twins, in the form tabular pseudohexagonal crystals, which were dark green in daylight, but changed to a purplish red when examined at night by a tungsten filament lamp. They were transparent, to semi-opaque when containing a high density of inclusions.  Most of the crystals were too dark for faceting of large stones over one carat, but much smaller ones of say 10 to 20 points would probably be quite bright.  Being a textbook example of cyclic twinning makes them excellent specimen material, so that even the opaque stones would have a market value.  It would be interesting to experiment by using a thin slice of this alexandrite to make doublets, perhaps cemented to a base of colorless synthetic corundum, to give a brighter stone in a larger size.

About a mile away on the same Novello claim is an occurrence of emerald, actually close by the track coming in.  Some excavations have been made but no extensive mining carried out.   The Girdlestone Ranch is indeed a gem hunter’s paradise; beside alexandrite and emerald there have been find of sapphire and the rare green garnet uvarovite.  Minor amounts of the lithium minerals petalite and lepidolite occur in the pegmatites. The emeralds occur in very distorted mica schist adjacent to the pegmatite veins and stringers. Local prospectors have found the small anthills useful as an indication of what lies below in the soil.  It would surely be a great thrill to find an emerald-studded anthill!

Cyril was an expert at spotting an emerald-bearing rock from a great distance.  Flat or straight schist was always barren, folded schist was worth examining, but most promising of all was the really contorted and knotted variety which invariable had tiny emerald crystals poking out here and there.  Large chunks sometimes had a quartz vein carrying whitish opaque beryl, while a few centimeters away in the knotted schist were tiny bright-green emeralds.  They particularly liked to form along the fold axes of the schist.  All the emeralds were flawed and more or less opaque, naturally of any gem value was found on the dumps, which had been inspected countless times.

This occurrence is similar to that of Sandawana, the big emerald mine operated by Rio Tinto, however there the country rock is a tremolite or actinolite schist. The Sandawana emeralds are noted for their tremolite fiber inclusions as well as their intense green color which allows cutting of fine colored stones down to 10 point size (1/10 carat).

To complete the day we decided to stop off on the way back and visit the old Twin Star Emerald Mine.  In former days this had been quite an extensive operation, complete with treatment plant now fallen into disrepair.  As the crow flies it is only 5 to 6 miles away.  Cyril, always a fund of local information, assured us that we could go by a new direct “road” through the bush and so cut off about 8 miles by not driving back onto the main road.  This we did, much to the consternation of poor old Don, who had visions of sleeping out in the wilds of Africa instead of the comforts of the Victoria Hotel.  I am sure Mr Hertz would have had apoplexy if he had seen his little Toyota plowing through waist high grass and across rocky creek beds.  There didn’t seem to be any well defined track at all.  Eventually we burst forth into a bare dirt patch with a few thatched huts around, causing much astonishment to an African woman and a couple of piccaninnies.

A high security fence surrounded the workings. A new lease had been taken out over the property by a Salisbury based gem dealer. (Chrystal Jewels Pvt. Ltd.) and some prospecting work was being done at the far end of the old open cut.  Five African workmen were trenching along a mica seam and washing the weathered schist in a screen over a tank of water.  They were recovering a few hundred grams of opaquish emerald each day.  It looked quite promising; they say where there is smoke there’s fire, green fire naturally!  At Twin Star the main open cut is in serpentine rock.  At several points in the walls there were pegmatite veins intruding upwards, up to 100 cm wide, and consisting mainly of quartz.  The emerald is confined to mica seams or lenses in the serpentine, just like the alexandrite at the Novello Prospect nearby.

And what can we learn from these occurrences?  Firstly, the prospector may be comforted in that here we have the classic example of beryllium-bearing solutions emanating from a pegmatite intrusion and picking up chrome from the mafic country rock (analysed 0.2% Cr) to form emerald.  Further away from the pegmatite the solutions become desilicified with the result that alexandrite crystallizes instead of emerald.  These observations are useful for the emerald prospector.   However, anyone who has had experience at emerald growing, particularly by the hydrothermal method, which is more akin to natural formation, that these observations are just the tip of the iceberg, so to speak. The great emerald mystery is there to be solved if you ask the right questions and figure out the correct answers.

The crystal grower would certainly wonder about what form the beryllium takes while migrating from the pegmatite into the country rock.  Is it transported as a chloride, fluoride, carbonate or whatever;  are the solutions acidic, neutral or alkaline; and what of the temperature?  They must be very special solutions to be able to mobilize the chrome in chromite, which is notoriously insoluble in aqueous solutions.

Another thing  that has always puzzled me as why do we not find naturally occurring beryls colored by nickel or manganese, which are trace elements more readily available than chromium (from chromite) and vanadium (from magnetite).  Nickel and manganese beryls should be more common than chromium emerald.  Hydrothermal experiments have shown that it is a simple matter to grow Ni, Mn and Co-doped beryls over a wide range of pH which is not the case with chromium, and yet very rarely do we find natural beryls having more than trace amounts of these elements, that is sufficient to influence the color.

It is even more puzzling to take a broader look at the situation.  Why are these beryllium solutions able to leave the confines of the pegmatite vein at emerald deposits, but are not noted for doing so at “normal” beryl-containing pegmatites?  A comparison of emerald producing pegmatites with beryl pegmatites shows that the latter usually have more complex mineral associations.  For example, the fabulous gem pegmatite of St Annes near the northern border of Rhodesia produces blue topaz, aquamarine and tourmalines. The schistose country rock contains is studded with staurolite and schorl, but no emerald.  The beryls are of pale pastel shades due to traces of iron, and they are always found in the pegmatite vein, not in the schist.

Emerald crystallization in Nature is compatible with the micas (phlogopite and biotite), tremolite-actinolite, albite, quartz, calcite, dolomite (Colombian deposits) and pyrite.  Emerald is not often found with tourmaline, and it seems to have a decided loathing for topaz.  Although the mineral association beryl-topaz is a common one for granite pegmatites, it is rather strange that you rarely find emerald and topaz together……the Australian Poona emerald occurrence being a possible exception.  My conclusions are that emerald is rather fussy about its mode of occurrence and who its mineral neighbors are; perhaps for geochemical reasons known to itself.  

Regards from Allano  (my ponderings on emeralds in 1976;  I’ve learnt a lot more about them since then, now 2019).

PET ROCKS DO TRAVEL

It is convenient to have a special box or container to carry around your collection of pet rocks, or crystals, for use and to show others.  About four in number is ideal and takes little space in one's shoulder bag.   Now you are set for any.contingency

What to pack?    You always must have a quartz crystal, also I always carry a topaz crystal, either  a colorless waterworn crystal, or a fractured crystal,  both from Brazil.   Next I have a grey-colored fossiliferous sandstone, rounded and beach-worn, having embedded white Turritella fossils.  My fourth item is variable, in the sense it may be one a number of things, like a beryl crystal, or danburite, or rose quartz.  What ever takes your fancy.

Each item represents a book of knowledge about itself.  If you have never seen a topaz before and have not read about one, then the topaz book is closed….so you may have the opportunity of opening the topaz book for someone, in various ways.   It may be by talking about topaz jewellery, or by remembering a line in a poem about stones  by the Chilean poet Pablo Neruda e.g.,   Cuando se toca el topazio,  el topazio te toca….etc  (When you touch the topaz, the topaz touches you) .  The topaz seems to be a live thing.

If you are a geologist, or gemmologist, you will know all about the properties and the history of origin of your pet rocks which you have learnt from a book. You can play with your memories, and you can help others not familiar with the subject.  How many people can identify a topaz crystal, or even a quartz?   Show them how.

Take you pet rocks on holiday with you.  For use as "handling stones" they have to fit comfortably in your hand.  When beachcombing, you may reject 20 or more stones in order to find the prefect one.  Ditto when buying in a shop.  Always have them with you.  They are nice to have when in hospital.  You can't take your dog, or cat,  to hospital, but you can take your pet rocks.    Regards from Allano.

SYNTHETIC QUARTZ WAND

I like to collect manmade gemstones and ornaments, having been much of my working life involved in their handling and synthesis. Often I browse through eBay to see what's on offer and occasionally buy something of interest. Recently I bought such an item, called a "Synthetic Quartz Wand", which was interesting to me, as I had frequently grown quartz crystals in the laboratory, and had got my chemisty students to grow them too. What was it going to be like?

It cost $10 so I was not expecting anything marvellous. What I did get was rather unique, and has become one of my favorite pet stones. At first glance you would say it's a beautiful clear quartz crystal of size 7.5cm long and 2 cm across, weight 50 grams, exhibiting hexagonal prism faces capped at each end by six pyramid faces of about equal size. Along one end was a lovely spray of wispy inclusions giving it a natural appearance.

What Oh, what have we got here, I wondered. I've never seen a synthetic quartz like this before. Normally they are flattish having been grown on basal plane seed plates. The first thing I did was to use a hand lens (10X to 40X) to examine the inclusions. They were tiny spherical gas bubbles all nicely swirling around…..and not to be found in a natural crystal, but frequently encountered in glass imitations. So I then tested it assuming it was glass, or an item you might find dangling from a chandelier.

An amateur collector may remain stymied at this point, lacking any testing equipment, and remain happy about it being a nice quartz crystal, suitable to use as a wand in crystal healing. It can still be used for this purpose, since crystal healing is I believe largely the power of positive thinking. However, it's nice to know what it really is. Using a gem refractometer, the R.I. was found to be 1.49 and isotropic, typical of glass, whereas quartz is doubly refractive with a mean R.I. of 1.55. The specific gravity was 2.4 compared to 2.65 for quartz.

If you examine a perfect natural quartz crystal, like a coffee table specimen, you will find the prism faces have weak horizontal striations, which on some may alternate with faces covered in etch marks. The termination of the crystal is with two sets of rhombohedra, giving a hexagonal pyramid-like capping, often with widely different sizes, with one face being dominant. It is possible for these natural features to be ground and polished off on a lap and so add to the confusion of identification.

I like my man-made glass wand and it takes its place among other pet stones and crystals, which include topazes, tourmalines and beryls.

Foto shows a nice tri-color tourmaline cluster from Brazil. The pink, and green, transparent and hopefully flawless gemstones, make for valuable ornaments, specially ring stones.

ON USE OF COMFORT STONES AND PET ROCKS

What are they? I am using the word STONE in its very general sense to include rocks, crystals, minerals, and gemstones which you may be fond of and consider your friends i.e., they are a comfort to you and give you pleasure to have and to hold. Hence they are small specimens of size about two inches across, or there abouts, and that can be readily held in the hand. So you can feel them and squeeze them and identify them even in the dark, from their shape, weight and other physical properties. They are great for when you are feeling downcast, and to take your mind off worrying things. Even should you go into hospital, where you can't take your pet cat or dog with you, but you can take a few pet rocks for comfort.

I don't know how the human brain works but I do know you can fool it by deliberately thinking about something pleasant or challenging, and so avoid or lessen one's thoughts on unpleasant things that you would prefer to forget. This is where your pet rocks come into play; so think about them, all about them, and you will be happier and more learned. Your brain will be activated on remembering useful and important things about them. I shall give you some examples of my comfort stones that I keep by my bedside always within reach; holding them helps me get to sleep; also to wake up if I should accidentally lie on one.

It helps to have a background knowledge in geology or gemmology, such as I have, but this knowledge is readily picked up by reading on the subject. Get a suitable book out of the library and read it, instead of idly watching TV or messaging with your mobile phone. Try learning something new for a change.

I have many favourite comfort stones. Here are six which are special and why I like them.

(1) A gin-clear quartz crystal, weight about 30 grams, showing all its crystal faces, but sawn across at the base so that it will stand up. I have several of these of various sizes and they are readily available. What can you learn from such a crystal? You can feel the six hexagonal prism faces that are capped to a point by six rhombohedral faces of differing size, often one face much larger than the others. In daylight the quartz crystal may show a conchoidal fracture some where, and weak horizontal striations on the prism faces. The crystal is quite colorless when having no impurities or crystal defects.. It has grown from a hydrothermal aqueous solution at modest depth in the earth; in a quartz vein in a quartzite, or pegmatite vein in granite terrain. Is it local or foreign? I have perfect ones from White Rock Quarry in the Adelaide Hills, one from Mt Antero in Colorado, and many from Brazil.

(2) A rounded fossiliferous sandstone of 70 grams weight. It is beach worn and was found on the local Brighton Beach, at Adelaide in South Australia. This is a rather plentiful source, so maybe you can easily find one locally too. It is a hard, rounded grey sandstone pebble just bristling with white turritella fossil shells (the pointy spiral ones) and their fragments. Here is a sample of sandy beach that existed nearby ages ago, but fairly recent geologically, less than 10 million years ago. This is a time capsule. Can you imagine that long ago and what the landscape was like, and what strange marsupial animals were hopping about? Such musings will keep you occupied for some time.

(3) An angular topaz crystal weighing 80 grams. It is flawless and perfectly transparent. It feels heavy compared to quartz, since their specific gravities are 3.5 and 2.65, respectively. It seems colourless but when put alongside a clear quartz crystal a faint blue tint is discernible. The trick to identifying topaz from quartz or other colourless crystals, is to find evidence of its perfect basal cleavage. Find one cleavage and you will find the other, opposite side. This is possible even on waterworn topaz, using a 10x lens. It has a conchoidal fracture elsewhere. Topaz has the same specific gravity as diamond but has a lesser refractive index (1.62 compared to 2.42 for diamond. This specimen is from a pegmatite cavity, unworn, but its mate is from a stream bed.

(4) Water-worn colourless topaz pebble of unknown locality. Weight 72 grams).
This is a completely colourless crystal and good faceting rough, if you want to imitate a diamond. Although water worn, tiny patches of the shiny basal cleavage surfaces are visible on two adjacent sides. Two flattish sides are remnants of prism faces. Although water-worn, its crystallographic orientation can be ascertained. it's as hefty as a diamond. I wish it was one. Retired faceters always have such topaz pebbles lying around, and they may remind you of earlier times when fossicking a stream for gemstones.

(5) Schorl, or black opaque tourmaline crystal, of weight 66 grams. Everyone should have tourmaline as an adornment, in a ring, bracelet or necklace, particularly a transparent coloured one. My special schorl crystal shows all the typical crystal faces, of trigonal prisms and pyramids. Schorls are heavy iron-rich tourmalines, being opaque black…you can't get anything blacker than schorl. It is a nice crystal to feel in the dark. It is a great friend.

(6) A clear "quartz crystal" bought on eBay, weight 50 grams, length 7.5 cm, width about 2.5 cm. and having hexagonal prism faces capped at each end by pyramid-like faces, all nicely polished. It has become one of my favorite stones, because it reminds me that things are not always what they seem at first sight. It looks like a colourless quartz crystal, like genuine quartz, but wait a moment, something is not quite right about it. I have never seen a doubly terminated quartz crystal like that before, with the pointy end faces so nearly all the same shape……and centrally there is a shower of wisps, very pretty too, which on examining with a 10x lens were revealed to be tiny gas bubbles, such as commonly seen in glass gemstone imitations. Yes, it is a glass imitation of a natural quartz crystal. Very clever. I like it. It was sold as "Synthetic Quartz Wand" for $10. I did not complain to the seller.

Other crystals and stones that are readily available and lend themselves as suitable comfort stones are beryl, other quartzes such as citrine, smokey, rose quartz (rough looks like candy), jaspers and agates, and there are the jades, and many fossils, in fact anything that takes your fancy and you want to be friends with. You will find your knowledge of minerals will be greatly enhanced by having a few pet stones to handle and cherish. Your peace of mind, and maybe your health, will show improvement too. Wishing you all the best of outcomes.

Regards from Allano. 07/04/2018

The Red Beryl Mystery

Natural red beryls are very rare and found in only one place in the world, that being in Utah, USA. Tiny specimens described as "Red Emeralds" were discovered in 1904 in fissures in a rhyolite lava flow in the Thomas Range (1). Later, in 1958 gem quality crystals were found in the Wah Wah mountains of Beaver Co., Utah. Since then small scale mining has been undertaken intermittently and supplied the gem market with cut and rough material, albeit of small size, much less than 1 carat size being the norm, and usually heavily included.

In 1971 the red crystals were identified for the first time by the Gemological Institute of America (GIA) and pronounced to be a red variety of beryl colored by traces of manganese (Mn), which also causes the red and pink color of the manganese minerals rhodochrosite and rhodonite, although in the beryl the manganese is present as a trace element with MnO less than 1.0 %. Analyses by others report MnO ranging between 0.18% to 0.82% and FeO ranging 1% to 3%.(2) The green color of proper emeralds is due to trace amounts of chromium or vanadium or both elements.

My interest in red beryl stems from reporting on the gem potential of synthetic cobalt beryl to the Gemological Society of G.B. in 1967, having grown in the laboratory some single crystals doped with cobalt and faceted a few of small size, about 0.5 carats (2). They had strong dichroism, purple to orange, and R.I average 1.565. The e-ray that was purplish red and showed a typical cobalt absorption spectrum with three dominant bands at about 545, 560 and 580 nm and appeared red through the Chelsea Filter. The o-ray, which was a pale orange in color, seemed devoid of the cobalt bands and so appeared green through the Chelsea Filter. This was observed using a measuring gemological spectroscope, which overall was a peculiar result. At that time (1967), I thought maybe in the future somebody or company will produce "red beryls" for the gemstone market, but you are not likely to make your fortune in doing so.

Recently I had reason to do a search on "cobalt beryl" and was surprised to find that even in the interlude to 2001 there have been recorded over 33 articles on the subject of red beryls and related doping of synthetic beryls with cobalt. What have I been missing out on?

From a gemological aspect, the most important and impressive is the GIA article "Hydrothermal Synthetic Red Beryl from the Institute of Crystallography, Moscow (3). The GIA scientists procured for study faceted red beryls up to 3.85 carat size and crystals up to 65 grams. The Russian method of growing beryl hydrothermally is a great advance over what we used in the 1965 by the Linde Company in USA. It was really an engineering problem. The Linde emeralds were grown in tiny platinum lined vessels of about 30 ml capacity. The engineering problem was how to scale up when the PT conditions were 600 degrees C and 500 to 1000 bars water pressure? The Russians solved the problem and I see that these red beryls were grown in stainless steel pressure vessels of 200 to 800 ml capacity; i.e., you need a reliable seal that doesn't leak after a week or more. The Russians get top marks for growing excellent crystals of red beryl and emeralds.

However, it is curious to note that the article title is "Hydrothermal Synthetic Red Beryl .... " and it is compared to the natural red beryl from Utah, whereas in fact it is a different "animal", being a cobalt beryl. The text says the crystals were grown with both manganese and cobalt to produce the red color. The analyses presented of the synthetic crystals gives consistently CoO ca 0.30% and MnO O.01 to 0.18%. Cobalt enters the structure mostly as Co2+ in the octahedral Al-site.

The appreciable ferrous iron present (FeO 1.32 to 1.62%) could originate from corrosion of the steel pressure vessel, or from the use of natural beryl nutrient, however it has minimal effect on the the color due to lack of strong absorption bands in the visible spectrum. On the other hand, analyses of natural red beryl do not contain cobalt and rely on manganese (MnO 0.18 to 0.82%) for the red color (chemical analysis does not specify whether Mn is 2+ or 3+).

A major supplier of synthetic crystals including hydrothermally grown beryls is Tairus Created Gems, which is a joint venture between the Russian Academy of Sciences (Siberian Branch) and Tairus (Thailand) Co. Ltd of Bangkok. Tairus grow for sale Red Beryl containing Co2+ and Co3+ and a Purple Beryl containing Mn3+, Co2+ and Co3+.

Data compiled at Cal Tech on beryl spectra (4)shows that manganese-containing beryls are pink when the manganese is present as Mn2+ (morganite) and red when Mn3+ is present (Utah naturals). Spectra is given for a synthetic pink beryl containing manganese. The absorption spectrum for natural red beryl, the e-ray gives a broad intense band from 450 to 600 nm, centered at 560 nm due to Mn3+. The o-ray is similar with the broad band centered at 545 nm. No cobalt bands are visible since there is no cobalt present.

I don't know how prevalent cobalt is in hydrothermally grown red beryls (great mystery), but if cobalt is normally added to the hydrothermal brew, then the detection of the cobalt absorption spectrum in the crystal makes for an easy way of telling the synthetic from the natural gemstone, which may be tricky in small gems. Maybe cobalt is added to make the crystal more red, than would occur with Mn2+ alone, rather than Mn3+ which seems to color the natural Utah crystals. The mystery prevails.

Some References
(1) "Red Emerald History", www.redemerald.com/history.html
(2) "Synthetic Cobalt Beryl" by A.M. Taylor, (1967) Journal of Gemmology, Vol. 10, No 8, pp.258-261
(3) "Hydrothermal Synthetic Red Beryl from the Institute of Crystallography, Moscow" by James E. Shigley et al., Spring 2001, Gems and Gemology pp.42-55.
(4) "Color in the Beryl Group; Beryl Visible Spectra (generally 350 - 2500 nm)" http://minerals.caltech.edu/FILES/Visible/BERYL/

Mexican Obsidian ... it makes the sharpest of knives!

The Pacific "Rim of Fire" is a region of volcanoes that are periodically erupting lavas and spewing ash into the atmosphere. When these lava flows cool quickly they freeze into a volcanic glass that is called obsidian. The lava may flow into the sea, or a lake, and thus cool quickly. Obsidians tend to be fairly high silica (70% +)of rhyolitic composition. Volcanic regions having occurrences of obsidian have been sought after by ancient man and used to fabricate arrowheads and knives. Obsidian makes the sharpest of knives and even today may be used by surgeons.

Snowflake obsidian depicted here is a favorite lapidary material for making pendants and bead jewelry. The lava has not cooled quickly enough to form all glass, but has allowed white cristobalite spherulites, a variety of silica, to nucleate and grow, and now appear within a glassy matrix.

A fortune teller's crystal ball?? No,a ball of silver sheen obsidian which is probably as good for telling fortunes. The sheen is said to be caused by aligned microscopic gas bubbles. The iridescent rainbow sheen obsidian opposite has been shown to be caused by inclusions of magnetite nanoparticles.

Mexico City is a good place to see and buy a good range of obsidian. The nearby ancient city of the Aztecs, Tiotihuacan, famous for its pyramids of the Sun and Moon, was once the center for obsidian workshops with dozens of shops producing obsidian knives, arrowheads, swords and ornaments. Now-a-days on tour you can visit some local museums or buy replica artifacts and obsidian specimens from the many stall holders.

The most famous location of old Aztec obsidian quarries is at Sierra de la Navajas (Hill of Knives) near Pachuca, not far away.

Another popular form of obsidian is that known as Apache Tears, which is found plentifully in Arizona. They are residual nodules of obsidian found within perlite rock which forms on weathering of the obsidian. These transparent tear-drop forms of obsidian are tumbled polished to make attractive pendants and other jewelry.

Amber from Chiapas State, Mexico

A stroll around the Zocalo in Mexico City, if you are observant, will reveal stalls selling cheap amber jewellery and trinkets. The amber comes from occurrences in the mountains of Chiapas State near the border with Guatemala. Much of it is pressed amber showing embedded fossilized insects, scorpions, and even peso coins, the latter should make you suspicious of their authenticity.

To learn more about Mexican amber you have to go and stay awhile in San Cristobal de las Casas, a southern city of population about 100,000 and located up in the mountains at an altitude of 2100 meters. From Mexico City it is a journey of 1085 kms and takes 19 hours by bus. There are several interesting places to visit on the way, like Puebla and Oaxaca. San Cristobal is a junction for travelers going to or from the Yucatan and Guatemala. It is a haven for backpackers, hippies and tourists with its cheap accommodation and plentiful supply of Indian handicrafts.

In San Cristobal one must visit the Museo del Ambar de Chiapas which is housed in the Ex Convento de la Merced to see a huge collection of local amber and learn the history of its exploitation. The ancient Mayans of southern Mexico used and traded amber. The present day mines are near Simojovel and Totolapa in Chiapas State, some 80 kms north of the city. It occurs in a grey, micaceous sandstone of late Oligocene to early Miocene Age (20 to 30 million years ago) with a capping of lignite. It originates from the resin of the tree "Guapinol" and is associated with fossil brachiopods, gasteropods and molluscs.