Emerald is used to celebrate the 55th Anniversary according to the World Almanac and Book of Facts. It is probably suitable that such a rare gem as emerald is used for this event as few events or happenings, aside from birthdays, reach this many years. Fine emeralds are probably among the rarest of gems. They are probably rarer than fine diamonds (the 60th Anniversary Stone) but diamonds more strongly signify endurance than emeralds.
Emerald is a green to blue green variety of the mineral Beryl, which has a slightly complex chemical formula:
Al2Be3(Si6O18)
a silicate of Beryllium and Aluminum. Crystals are hexagonal, 7.5 to 8 hard, 2.65 to 2.75 in specific gravity. Emeralds, as all other beryls, can range from being opaque to transparent.
Beryl has fairly low optical properties. The refractive index ranges from 1.57 to 1.60 with many gems falling in the 1.572 - 1.580 range. Beryl has a low birefringence and are only weakly doubly refractive. It would take a piece of beryl about 15 inches thick to show a double image. The dispersion of beryl is also quite low (0.014) and emerald, as other beryl gems, has to essentially make it on color and pleasing appearance without the brilliant flashes of light that we associate with diamonds or gems with a very high refractive index or dispersion.
When one is looking at beryl gems, there is really no firm boundary between emeralds with a heavy blue tint or aquamarine with a heavy green tint and there is no firm boundary between the green to blue green of emerald and the green of green beryl. This might have been problematical in years past; however, there are now standardized color sets that are available from Gemological Institute of America that every reputable jeweler should have as a tool of the trade.
Emerald owes its color to a small percentage of Chromium Oxide in the crystal lattice---up to 3%. Strangely, this is the same oxide in the same percentage that imparts the red color to ruby when it is in the corundum lattice.
In my gemology classes at the University of Nebraska, I utilize a small color set. These are available from GIA and are produced by Pantone. I suggest that the bG 6/4 or the vslbG 6/4 colors are the only two in the set that represent emerald. The vstbG 5/5 is too blue (it would be a superb aquamarine) and the G 5/5 hasn't enough blue (it would be a superb green beryl).
Emeralds are quite rare because of the rather limited geologic environment in which they can occur. Most emeralds form in contact metamorphic rocks---that is, the narrow, baked zone where a hot magma (lava) comes into contact with sedimentary rocks such as limestone or shale. Many emeralds come from contact metamorphosed black shale beds. Black shale is often the thinnest bed that is in a sedimentary rock sequence so there is little potential host rock for starters. Some emeralds form in limestone that has been subjected to contact metamorphism.
Emeralds are usually clouded by many inclusions. The nature of the inclusions is usually indicative of the source of the emerald. Emeralds from Colombia normally have three phase inclusions. These consist of a solid (usually a pyrite crystal), a liquid (salt water?) and a gas (usually CO2). Emeralds from India usually have "square," dark inclusions (biotite) whereas those from South Africa have dark needle-like inclusions and those from the Ural Mountains in Russia have actinolite needles that resemble bamboo poles.
In the United States, few emeralds have been found and most have been recovered from near Hidden, North Carolina. A very large emerald that became the subject of a bitter law suit was mined there in the 1980's. The tragi-comic outcome was that one of the miners involved sawed the very fine 14 inch crystal (valued at several millions of dollars) in half and destroyed much of the value of this essentially priceless item. Except for the North Carolina finds, no other state has had any significant emerald production.
Almost all emerald is mined from in situ localities or deposits that are very close to the mother lode. This is because the emerald is a very weak stone---it will not endure the abuse and rigor of transportation in streams or in glacial ice. This observation does not rule out micro-emeralds as one of the local collectors recently brought in an example of a micro-emerald crystal that was collected from sand in the Dakota Group of late Cretaceous age in southeastern Nebraska.
Australia has been a fairly important producer of emeralds. Several important sites in New South Wales and Western Australia have been described. Many of the emeralds that have been shown in colored prints are probably closer to green beryl but some fine blue green stones are known.
The low physical and optical properties of emerald make it easy to separate from other gems such as peridot (very strong double refraction); tourmaline (strong double refractions and thready inclusions); tsavolite garnet (singly refractive); and glass (bubbles, swirl marks, etc.).
Separating synthetic from natural emeralds is a bit more difficult. There are many would be tests that are non-diagnostic and one has to be careful. Synthetic emeralds are usually produced by some hydrothermal process in which beryl is caused to recrystallize from a saturated, heated melt or solution in a bomb. The synthetic will usually contain tiny crystals of the flux that was used in the solution, or it may contain micro-phenakite crystals or micro-platinum crystals. Inclusions that are wispy or cob-web like also are seen in many synthetic emeralds.
Carroll Chatham, the first person to make synthetic emeralds was only 15 years old when he developed the process. Chatham had a love for chemistry and did his first experiments with explosives. After destroying his laboratory, his father ordered research of a less violent nature. Other companies have followed and most synthetic stones are now called "created" stones.
References
- Bauer, M., 1896. Precious Stones. A popular account of their characters, occurrence, and applications, with an introduction to their determination, for mineralogists, lapidaries, jewellers, etc. Translated by Spencer, L. J., 1904. Reprinted by Charles E. Tuttle Company: Publishers, Rutland, Vermont & Tokyo, Japan, 647 p.
- Myatt, B., (Editor), 1972. Australian and New Zealand Gemstones. Paul Hamlyn Pty. Ltd., Sydney, 511 p.
- Perry, N. & R., 1967. Australian Gemstones in Colour. Charles E. Tuttle, Rutland Vermaon and Toky, 112 p.
- Schumann, W., 1977. Gemstones of the World. Sterling Publishing Co., New York, 256 p.
- Shipley, R. M., 1971. Dictionary of Gems and Gemology. Gemological Institute of America, Los Angeles, CA, 227 p.
- Sinkankas, J., 1959. Gemstones of North America. Van Nostrand Reinhold Co., New York, 675 p.
