Emerald
...n atoms in a tetrahedral arrangement. A remarkable feature of the emerald is that the Si-O rings are aligned exactly over each other so that their openings form continuous channels parallel to the c axis. The uniformity and abundance of these bonds account for its hardness. Consult Fig 1.1 for further clarification. (Sinkankas and Read, 1986). Mohs scale classifies emerald under a hardness of 7.5-8 (Read, 1991). Regardless of the uniformity and abundance of these bonds, it is still a brittle gemstone. Emerald’s imperfect poor basal cleavage contributes to this property. Cleavage is caused by weakness in planes of the crystal structure. Due to the emerald’s compact structure, with most bonds arranged in continuous channels parallel to the c axis, it can be easily cleaved with confidence at 90 degree angels to the c axis. In fact many ancient artefacts take advantage of this property, Indians created ornamental flat stones while Mogul jewellery had flat sections in the typical engraved plaques (Sinkankas, 1981). Specific gravity is a measured value derived from the weight of a specific material divided by the weight of the same volume of water. Emerald’s specific gravity ranges generally between 2.7-2.8 (Read, 1991). Another physical property that identifies emerald is its refractive index of 1.56-1.59 (Read, 1991). There are several tests that enable one to identify principle characteristics of an emerald. If placed under a black light an emerald will not glow, this indicates the emerald possesses a non-fluorescence luminescence. The colour of the powder left behind when an emerald is scratched against a streak plate should be white. The naked eye is able to observe the vitreous resinous lustre while a dichroscope may aid in determining the dichroism of yellow-green or blue green (Dana, 1857). Plate tectonics are responsible for creating the intricate patchwork of oceanic rocks, which contain traces of chromium and vanadium, colliding with continental rock filled with beryllium. These elements combined in high temperature liquid usually come together in the formation of new mountain ranges. As the edges of the two colliding plates stack upwards, continental rocks can be pushed deep into the Earth’s surface that they will melt again, creating a huge balloon of magma to swell upwards towards the crust. Upon reaching its neutral buoyancy the magma will cool naturally and solidify into granite. Less than five percent of the granite creates a remarkable gemstone. If by chance the felsic granite happens to contact a zone of mafic rock incorporated in the continental crust, then the chemistry will create an interesting treasure. The felsic-mafic combination will generate a fissure filled with biotite, a type of black flakey useless mica. However in this black worthless rock, scattered like green snowflakes, may be emeralds. Emeralds are usually found in an environment of granitic pegmatites (Kunziq, 1999). The finest emeralds are found in Colombia embedded in a dark bituminous limestone. Columbian emeralds can be easily identified by its three phase intrusions with jagged ends, containing a liquid, a gas bubble, and a salt crystal (Read, 1991). Geologists were surprised to find the absence of granite in this famous location of emeralds. Instead of granite intruding from below, black shale are laid down from above. Dispersed within the Columbian shales contain all the ingredients for the creation of an emerald (Kunziq, 1999). The most infamous mines are in Muzo and El Chivor. Muzo is unique due to its freak paragenesis where emeralds are found in calcite veins in the black bituminous limestone. Emeralds also occur in the almost pegmatite veins at El Chivor (Pough, 1996). Muzo has a more yellowish emerald than Chivor (Sinkankas, 1981). Another difference between the two emeralds is their characteristic inclusions. Muzo’s emeralds are identified by yellow/brown rhombs of parasites and rhombs of calcite while Chivor’s emeralds are identified by inclusions of albite and pyrite crystals (Read, 1991). Both mines have been under control of private and government ownership at various times. Regardless of ownership, stealing, bribery, murder have always accompanied the mining operation. Presently mining at nationalized mines is carried out under armed guard. These problems still exist and a large percentage of emeralds are still sold illegally today (Klein and Hurlbut, 1912). It is understandable for these highly valuable gemstones to be mined under armed guard. In 2000 the auctioneers Christies, sold a 10 carat emerald for over a million dollars, this is equal to approximately $100,000 per carat. The price range for a carat of emerald ranges depending on the quality of the gemstone. Regardless of size, brilliant clear emeralds are rare and expensive, but commercial grade emeralds are plentiful. One carat emerald commercial grade ranges from $30.00 to $525.00, good quality ranges from $525.00 to $1,125.00, fine quality can be priced from $525.00 to $1,125.00 while extra fine quality can reach prices from $2,900.00 to 9,800.00 (Mineral Gallery, 2005). An interesting natural phenomenon is the growth of a trapiche emerald. The trapiche emerald is in a class of its own. Trapiche is a Spanish word that normally refers to cog-wheels used to crush sugar cane. Thi...