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In geology, a metamorphic zone is an area where, as a result of metamorphism, the same combination of minerals occur in the bedrock. These zones occur because most metamorphic minerals are only stable in certain intervals of temperature and pressure .
Clarity grading in coloured gemstones is based on the eye-clean standard, that is, a gem is considered flawless if no inclusions are visible with the unaided eye (assuming 20/20 vision). [29] The Gemological Institute of America classifies tanzanite as a Type I gemstone, meaning it is normally eye-clean.
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While gem beryls are ordinarily found in pegmatites and certain metamorphic stones, red beryl occurs in topaz-bearing rhyolites. [53] It is formed by crystallizing under low pressure and high temperature from a pneumatolytic phase along fractures or within near-surface miarolitic cavities of the rhyolite.
Anyolite is a metamorphic rock composed of intergrown green zoisite, black/dark green pargasite and ruby. [1] It has been found in the Arusha Region of Tanzania and in Austria. [1] It is sometimes incorrectly advertised as a variety of the mineral zoisite. The term anyolite is, however, not an officially accepted term for a metamorphic rock. [2]
Metamorphic facies is a classification system first introduced by Pentti Eskola in 1920 to classify particular metamorphic mineral assemblages that are stable under a range of P-T conditions. [ 54 ] [ 55 ] [ 1 ] Before the mid-1970s, geologists utilized the metamorphic facies classification to investigate metamorphic rocks and determined their ...
Zoisite occurs as prismatic, orthorhombic (2/m 2/m 2/m) crystals or in massive form, being found in metamorphic and pegmatitic rock. Zoisite may be blue to violet, green, brown, pink, yellow, gray, or colorless. Blue crystals are known under the name tanzanite. It has a vitreous luster and a conchoidal to uneven fracture. When euhedral, zoisite ...
It is often found in minute micaceous aggregates (with individual plates barely visible), as a major component of chromium rich phyllitic or schistose metamorphic rocks of the greenschist facies. Verdite is a type of metamorphic rock made mostly of an impure, often multicolored, variety of fuchsite. It is used for ornamental carvings.