Fake gem rough
27th Mar 2015 09:24 UTCDuncan Miller
This is a sample of parcel of 'emeralds' ostensibly from Zambia, ranging from 10-15 mm in length. Most of them are coated with biotite mica and have striations along the length. Superficially they are pretty convincing.
With some of the 'stones' turned to face the viewer you can see several tell-tale features. Emerald crystals are hexagonal, not pentagonal, so when lying on a prism crystal face a face and not a ridge should be facing up. You can see indentations on the 'ends' of the 'crystals', and the majority of the specimens have them. Natural emerald crystals, unless the ends are broken, have flat faces, not rounded indentations.
Seen end on, the pentagonal cross sections and indented 'pinacoids' are obvious.
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A close-up view through the polished face of one specimen shows the swirls of numerous bubbles typical of glass. These fakes are made from glass poured into mica-lined molds, and the indented ends are due to contraction as the glass cooled. No RI determination is needed to identify these as fake, but for interest it is 1,520.
This is a photograph of a 25 mm long 'terminated tourmaline crystal' bought by a customer of mine at the Klein Spitzkopje in Namibia. Apart from having eight sides (tourmaline commonly has three or six) it has abraded faces and a couple of small round bubbles (mid lower left). Through crossed polarizers the 'crystal' clearly was optically isotropic.
How about some chunky 'aquamarine crystals', obligingly covered in dirt, bought by a geologist (!) in Mocambique? The largest is 60 mm long.
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Here is another view of the 'aquamarine', showing that they are not hexagonal, but rather irregularly pentagonal in cross section. Beryl crystals typically are hexagonal in cross section.
A closer view of the end of one specimen shows strongly choncoidal fracture and round bubbles below the surface. These are chunks of greenish glass, with roughly ground and battered faces, lightly coated in reddish soil. They display no dichroism and between crossed polarizers they are optically isotropic.
This 35 mm diameter nodule was part of a parcel that did the rounds of jewellers in Cape Town, offered to them variously as ruby or garnet. I begged this example off a friend of mine for bursting his eager bubble. Anyone who has handled ruby or garnet gem rough would be highly suspicious of the size, the roughly abraded edges of this supposedly hard material, and the obscuring pebbly matrix.
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A closer view shows the large, conchoidal fracture surface, the heavily abraded exposed edges, and some round bubbles (top right). This too is glass, coated with an artificial 'matrix' of sand grains and pebbles in resin.
This is a 'matrix specimen' of material that the vendor claimed himself to have seen being dug out of the ground in Mocambique. It was accompanied by large transparent pieces, claimed to be epidote. It was not immediately obvious what this was, although it certainly didn't look like epidote.
The adhering brown crystals made this look even more like a natural mineral specimen.
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Here is a photograph of the clear material mentioned above, that was offered as epidote. It is about 25 mm long. The top face is a saw cut, but the front face and the side faces are cleavage faces, at 90 degrees to each other. It has cubic cleavage, so clearly is not epidote (or peridot, which it more closely resembles). There are swarms of minute round bubbles, which you can't see in the photograph, and it is optically isotropic but the cleavage ruled out glass. This was a tricky one to identify positively and I had to measure both the RI (1.73) and SG (3.55) to nail it. The RI and SG are consistent with synthetic periclase (MgO), made in a furnace and doped to make it green. The supposed 'matrix' specimens shown above were part of the furnace lining.
Parts of furnace linings can look deceptively like natural mineral specimens. This is a photograph of part of the lining of a clear glass tank. The refractory lining makes up the background 'matrix', with clear pieces of glass adhering to it. Without closer inspection this easily could be mistaken for a rather battered quartz on matrix specimen.
I am concerned about this mineral here. I recently purchased it. The man I got it from said it is a Muzo Emerald. However it appears that there is a small 7th side on this gemstone, which I thought to be inconsistent with the traditional 6 sided Emeralds I've seen. The "extra" face is very skinny and is located in between where the 2 faces meet on the bottom of the first photo. I have other Beryl crystals that have inverted flat extra faces on them, but I never seen an emerald like this. Please any help would be appreciated. do you think it is a real emerald?
