Saturday, November 1, 2014

Chrysoberyl; an elusive gemstone in Australia


This is a self cut chrysoberyl of 2.25 carats and size 8 x 5 mm made from Brazilian rough alluvial material. I wonder why chrysoberyl is (apparently) not found in Australia? Maybe it does occur in our gem gravels but is not recognized or is mistaken for something else?

Chrysoberyl is typically a honey yellow to pale
greenish colour and sometimes is a cat's eye variety. Composition is BeAl2O4, a beryllium aluminate, so it would occur with other beryllium minerals like beryl and phenacite in granite terrains .It is very hard (Moh 81/2) and has an SG of 3.7. My cut stone has an RI of 1.74 - 1.75 (DR ca 0.01) which is close to that of sapphire(1.76 - 1.77) with which it could be confused. Chrysoberyl is found in the gem gravels of Brazil, Sri Lanka and Zimbabwe and..... why not Australia?

A mineral collector has pointed me in the direction of mindat.org as a source of information on the matter. This is a huge database to do with minerals which has developed over the past 13 years. Yes, there are many recorded occurrences of chrysoberyl, particularly from pegmatites of the Harts Range, Northern Territory. These are small (to 1 inch)twinned crystals, specimen material, not facet grade. However, clear water worn pebbles of chrysoberyl are listed for the Weld River, Blue Tier district of Tasmania and look promising as gemstones. Gemstone hunters of the region should carefully study their haul of heavies from alluvial prospecting activities. The heavies on one's sieve would include gemstones with a Specific Gravity over 3.0, such as diamond, zircon, topaz, garnet, sapphire, ruby, spinel, tourmaline and maybe CHRYSOBERYL. Be alert and curious, and you might find one!