Identifying Raw, Uncut Emeralds
Unlike faceted emeralds, identifying uncut emeralds is much easier for a number of reasons. An emerald’s habit, the shape the crystal ideally grows as, is a hexagonal prism. If the rough emerald shows this growth characteristic without signs of being polished this way, as well as pieces of the rocks that they naturally form on (calcite is one example), it is very likely you are looking at a natural emerald.
As good as an identifying characteristic this is, it is an ideal formation. This is not the way emeralds grow every time. Nature almost never makes anything uniform, instead mixing up whatever random elements are in the ground at a given time and letting it cool in changing conditions over the course of millennia.
As a result, the exact appearance of the emeralds is somewhat random. Finding any specimens of higher quality is akin to winning the lottery. Exceptionally large gems can potentially go to auction, though the exact quality of what is inside is not completely known until the gem cutters begin faceting these stones.
There are a few basic tests to check. One of these is the streak test. The gem in question is dragged across an unglazed porcelain plate, and the color of the streak tells you what the true color of the gem is. This test will never be used on faceted gems because it is destructive, removing a small part of the gem and scratching the polish. Only unfaceted gems will be tested like this.
This test also has a few caveats. The porcelain will only scratch gems that have a hardness of 6 or lower on the Mohs scale, though there are ways to test gems of higher hardness. Part of a harder gem is to be pulverized and then smeared on the plate. It is imperative that only the gem is analyzed on the plate and not bits of another rock still attached to the gem. This means emeralds, with their high hardness of 7.5-8 on the Mohs scale, can be tested with some extra effort.
Streaks left behind by emeralds will not be green, but colorless. The stuff that colors emeralds, chromium and sometimes vanadium, is not a key part of its chemical structure. They are trace elements, or impurities, that were mixed into the beryl during formation in very small amounts.
Eye-visible inclusions are almost always present in emeralds. While most inclusions in emeralds are not diagnostic in differentiating them from synthetic emeralds without experience, the inclusions combined with rough shape is usually diagnostic for identification. Things like three phase inclusions and crystal inclusions serve as guarantees that these are natural emeralds.
The main caveat to inclusions is that aside from needing a loupe , the surface of the emerald can be cloudy and difficult to see into. Most cutters and buyers will polish windows onto these emeralds to get a better idea of the clarity, as well as figure out how they want to cut the gem.
Most individuals who buy rough gems will use a combination of techniques to quickly verify these gems, though emeralds can occasionally be identified at a glance.
Do beware large, clear emeralds that are dirt cheap. An emerald over a carat in weight that is perfectly clear will have accompanying reports from one or more accredited gem labs that can be verified before purchase. If you are looking for a synthetic emerald on the other hand, just make sure the seller is not overpricing the material.