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Topaz


Topaz

a group of topaz crystals on matrix
General
	Category	Silicate mineral
	Chemical formula	Al₂SiO₄(F,OH)₂
Identification
	Color	Clear (if no impurities), blue, brown, orange, gray, yellow, green, pink and reddish pink.
	Crystal system	orthorhombic
	Cleavage	[001] Perfect
	Fracture	conchoidal
	Mohs Scale hardness	8
	Luster	Vitreous
	Refractive index	nα = 1.606 - 1.629 nβ = 1.609 - 1.631 nγ = 1.616 - 1.638
	Optical Properties	Biaxial (+)
	Birefringence	δ = 0.010
	Pleochroism	Weak in thick sections
	Streak	white
	Specific gravity	3.49 - 3.57
	Diaphaneity	Transparent
	Other Characteristics	Fluorescent, short UV=golden yellow, long UV=cream

Topaz is a silicate mineral of aluminium and fluorine with the chemical formula Al₂SiO₄(F,OH)₂. Topaz crystallizes in the orthorhombic system and its crystals are mostly prismatic terminated by pyramidal and other faces, the basal pinacoid often being present. It has an easy and perfect basal cleavage. The fracture is conchoidal to uneven. Topaz has a hardness of 8, a specific gravity of 3.4–3.6, and a vitreous luster.

Color and varieties

Pure topaz is colorless and transparent but is usually tinted by impurities; typical topaz is wine or straw-yellow, pale gray, or pink. They may be made white, gray, pale green, blue, pink or reddish-yellow and transparent or translucent.

Yellow topaz is the traditional November birthstone, the symbol of friendship, and the state gemstone for the US State of Utah.

Blue topaz is the December birthstone and Texas state gemstone. Topaz very rarely occurs blue naturally. Typically, colorless, gray or pale yellow material is heat treated and irradiated in order to turn it blue.

Mystic topaz is colorless topaz which has artificially treated with a thin film/coating giving it the desired rainbow effect, and is not a naturally occurring topaz.^[6]

Localities and occurrence

Topaz is commonly associated with silicic igneous rocks of the granite and rhyolite type. It typically crystallizes in granitic pegmatites or in vapor cavities in rhyolite lava flows like those at Topaz Mountain in western Utah. It may be found with fluorite and cassiterite. It can be found in the Ural and Ilmen mountains, Afghanistan, Sri Lanka, Czech Republic, Germany, Norway, Pakistan, Italy, Sweden, Japan, Brazil, Mexico, Flinders Island and the United States.

Some clear topaz crystals from Brazilian pegmatites can reach boulder size and weigh hundreds of pounds. Crystals of this size may be seen in museum collections. The Topaz of Aurungzebe, observed by Jean Baptiste Tavernier measured 157.75 carats.

Colorless and light-blue varieties of topaz are found in Precambrian granite in Mason County, Texas within the Llano Uplift. There is no commercial mining of topaz in that area.

History, Myth & Lore

The name "Topaz" is derived from the Greek Τοπάζιος (Τοpáziοs), the author of one of the first systematic treatises on minerals and gemstones dedicated two chapters on the topic in 1652. In the Middle Ages the name Topaz was used to refer to any yellow gemstone, but now the name is only properly applied to the silicate described above.

Many modern English translations of the Bible, including the King James Version mention Topaz in Exodus 28:17 in reference to a stone in the Hoshen: "And thou shalt set in it settings of stones, even four rows of stones: the first row shall be a sardius, a topaz, and a carbuncle: this shall be the first row."

However, since these translations as topaz all derive from the Septuagint translation topazi[os], which as mentioned above referred to a yellow stone that wasn't Topaz, probably chrysolite, it should be borne in mind that Topaz is not meant here. The masoretic text (the Hebrew on which most modern Protestant Bible translations of the Old Testament are based) has pitdah as the gem the stone is made from; pitdah is of unknown meaning, though scholars think it is related to an Assyrian word meaning flashed. There is a wide range of views among traditional sources about which tribe of the Israelites the stone refers to.