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).