Green Natural Beryl Emerald GemStone
Beryl minerals occur in granites and granite pegmatites, in mafic metamorphic rocks and in high-temperature hydrothermal veins and in vugs in rhyolite. The Colombian emerald fields are located in the Cordillera Oriental, covering parts of the provinces of Boyaca, Cundinamarca and Santander. The important deposits at Muzo are Cretaceous (Sinkankas, 1981, op. cit.) and the mines lie in a series of black shales. It is interesting to note that while some varieties of beryl occur in granite pegmatites, this scarcely ever seems to be the case with emerald.
Emerald is one of the major gem species and one in which it is possible sometimes to identify the place and conditions of origin from a study of its inclusions. Chromium must be present if a green beryl is named as emerald; green beryl coloured by vanadium alone (these tend towards apple-green) should be termed green beryl. Gem-quality emerald normally has hardness in the range 6.5–8, and while there is no cleavage, specimens are notably brittle and should never be subjected to ultrasonic cleaning. The specific gravity of emerald as well as its refractive index varies with place of origin: Gübelin in Gems & Gemology 18(3) 1982 published a list of these figures.
Beryl is uniaxial negative and the low dispersion, at 0.014, does not enhance specimens Pleochroism shows most strongly in darker-coloured stones, emerald showing yellowish green and blue-green. Chromium-rich specimens show red through the Chelsea filter, though with increasing iron content the effect diminishes and some specimens from South Africa and India, in particular, show little if any change. Synthetic emeralds glow a pronounced red.