The identification of emeralds is an arduous task, which is said because it can simulate the growth conditions of natural emeralds under laboratory conditions and synthesize hydrothermal emeralds that can be almost confused. In addition, in Australia, this inferior emerald has been artificially improved in the laboratory to make it become a higher-grade emerald, that is, the emerald in the refining pool. Whether this emerald is natural or synthetic is still controversial. Emerald is composed of Be3Al2[SI6O 18], which is a silicate mineral of beryllium and aluminum. Its green color is usually caused by chromium or vanadium, and the content of Cr2O3 varies from 0.15% to 0.6%, generally in the form of hexagonal columnar crystals, with hardness of 7.5-8 and high brittleness. It has no reaction to other acids except hydrofluoric acid. Emeralds from all over the world often have some subtle changes in physical and optical properties. Other natural gemstones similar to emeralds are jadeite, garnet, tourmaline, fluorite and green zircon. In many cases, it is necessary to use small identification instruments such as refractometers and polarizers to distinguish these gems strictly according to their different characteristics (Table 2-8); From the experience, jadeite often has a variegated structure with interwoven fibers; Garnet has a large proportion and a ponytail-like inclusion; Fluorite has low hardness and high specific gravity; Zircon has strong dispersion and obvious ghost. These characteristics can be used as a reference for naked eye observation and identification.
Synthetic gemstones similar to emeralds mainly include synthetic emeralds, YAG (yttrium aluminum garnet) and green glass. Among them, the distinction between natural emeralds and YGA and glass can be based on the following two points:
1. Most natural emeralds have flaws and cracks, similar to rubies. Defects are mostly "cotton wool"-like, and sometimes some impurities such as black mineral inclusions can be seen. Synthetic products are mostly clean, or there are a few bubbles.
1) Please refer to ruby interlayer for the identification of interlayer stone. (2) Foil-lined emeralds mainly appear in some jewelry with bottom seal, that is, the bottom seal of gemstones. Because the bottom of the gem is covered with a layer of green foil, the green color of the gem will deepen and improve from the table. Therefore, all ornaments with back covers must be carefully watched.
(3) Oiling is the most commonly used method to treat emeralds. Because emeralds are brittle, there are often cracks and flaws. When some oil is injected into the fracture, the fracture is not so obvious. Cedar oil (refractive index 1.575) and Canadian gum (refractive index 1.52) are commonly used in emerald oil treatment. To check whether the gem has been treated with oil, you can carefully observe some surface cracks with a magnifying glass of 10 times. Oiled cracks will produce iridescence or some un-oiled cracks can be seen from different observation angles. In addition, put the gem under the electric light to "barbecue" (be careful not to be too hot)! Emerald with oil
Some gems will have "dirt", that is, oil will seep out along the cracks, and you can see the oil stains by wiping them with soft paper (observe them under a magnifying glass of 10 times). When testing whether oil is added, it is more important to pay attention to whether there is color at the same time. If it is found that the color is distributed along some cracks, it is abnormal.
4. Most of the key natural emeralds have flaws or inclusions, so it is best to have an appraisal book marked "natural" when buying valuables.
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