Topaz

Topaz
Topaz crystal on white matrix
General
Category Nesosilicate minerals
Formula
(repeating unit)		 Al2SiO4(F,OH)2
Strunz classification 9.AF.35
Crystal system Orthorhombic
Crystal class Dipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space group Pbnm
Unit cell a = 4.65 Å, b = 8.8 Å,
c = 8.4 Å; Z = 4
Identification
Color Colorless (if no impurities), blue, brown, orange, gray, yellow, green, pink and reddish pink
Crystal habit Prismatic crystals with faces striated parallel to long dimension; also columnar, compact, massive
Cleavage [001] Perfect
Fracture Subconchoidal to uneven
Mohs scale hardness 8 (defining mineral)
Luster Vitreous
Streak White
Diaphaneity Transparent
Specific gravity 3.49–3.57
Optical properties Biaxial (+)
Refractive index nα = 1.606–1.629
nβ = 1.609–1.631
nγ = 1.616–1.638
Birefringence δ = 0.010
Pleochroism Weak in thick sections X = yellow; Y = yellow, violet, reddish; Z = violet, bluish, yellow, pink
Ultraviolet fluorescence Short UV=golden yellow; Long UV=cream
References [1][2][3][4]

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. It is one of the hardest naturally occurring minerals (Mohs hardness of 8) and is the hardest of any silicate mineral. This hardness combined with its usual transparency and variety of colors means that it has acquired wide use in jewellery as a cut gemstone as well as for intaglios and other gemstone carvings.[5]

Characteristics

Facet cut topaz gemstones in various colors
Yellow topaz in stepped kite-shaped cut

Topaz in its natural state is a golden brown to yellow, a characteristic which means it is sometimes confused with the less valuable gemstone citrine.[5] A variety of impurities and treatments may make topaz wine red, pale gray, reddish-orange, pale green, or pink (rare), and opaque to translucent/transparent. The pink and red varieties come from chromium replacing aluminium in its crystalline structure.

Orange topaz, also known as precious topaz, is the traditional November birthstone, the symbol of friendship, and the state gemstone of the US state of Utah.[6]

Imperial topaz is yellow, pink (rare, if natural) or pink-orange. Brazilian imperial topaz can often have a bright yellow to deep golden brown hue, sometimes even violet. Many brown or pale topazes are treated to make them bright yellow, gold, pink or violet colored. Some imperial topaz stones can fade on exposure to sunlight for an extended period of time.[7][8]

Blue topaz is the state gemstone of the US state of Texas.[9] Naturally occurring blue topaz is quite rare. Typically, colorless, gray or pale yellow and blue material is heat treated and irradiated to produce a more desired darker blue.[8]

Mystic topaz is colorless topaz which has been artificially coated via a vapor deposition process giving it a rainbow effect on its surface.[10]

Although very hard, topaz must be treated with greater care than some other minerals of similar hardness (such as corundum) because of a weakness of atomic bonding of the stone's molecules along one or another axial plane (whereas diamonds, for example, are composed of carbon atoms bonded to each other with equal strength along all of its planes). This gives topaz a tendency to fracture along such a plane if struck with sufficient force.[11]

Topaz has a relatively low index of refraction for a gemstone, and so stones with large facets or tables do not sparkle as readily as stones cut from minerals with higher refraction indices, though quality colorless topaz sparkles and shows more "life" than similarly cut quartz. When given a typical "brilliant" cut, topaz may either show a sparkling table facet surrounded by dead-looking crown facets or a ring of sparkling crown facets with a dull well-like table.[12]

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 including those at Topaz Mountain in western Utah and Chivinar in South America. It can be found with fluorite and cassiterite in various areas including the Ural and Ilmen mountains of Russia, in Afghanistan, Sri Lanka, Czech Republic, Germany, Norway, Pakistan, Italy, Sweden, Japan, Brazil, Mexico; Flinders Island, Australia; Nigeria and the United States.

Brazil is one of the largest producers of topaz,[13] 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 Aurangzeb, observed by Jean Baptiste Tavernier weighed 157.75 carats.[14] The American Golden Topaz, a more recent gem, weighed a massive 22,892.5 carats. Large, vivid blue topaz specimens from the St. Anns mine in Zimbabwe were found in the late 1980s.[15]

Colorless and light-blue varieties of topaz are found in Precambrian granite in Mason County, Texas[16] within the Llano Uplift. There is no commercial mining of topaz in that area.[17]

Etymology

The name "topaz" is usually derived (via Old French: Topace and Latin: Topazus) from the Greek Τοπάζιος (Τοpáziοs) or Τοπάζιον (Τοpáziοn),[18] from Τοπαζος, the ancient name of St. John's Island in the Red Sea which was difficult to find and from which a yellow stone (now believed to be chrysolite: yellowish olivine) was mined in ancient times; topaz itself (rather than topazios) was not really known before the classical era. Pliny said that Topazos is a legendary island in the Red Sea and the mineral "topaz" was first mined there. Alternatively, the word topaz may be related to the Sanskrit word तपस् "tapas", meaning "heat" or "fire".[18]

Historical usage

Nicols, the author of one of the first systematic treatises on minerals and gemstones, dedicated two chapters to the topic in 1652.[19] In the Middle Ages, the name topaz was used to refer to any yellow gemstone, but in modern times it denotes only the silicate described above.

Many modern English translations of the Bible, including the King James Version mention topaz. However, because these translations as topaz all derive from the Septuagint translation topazi[os], which as mentioned above referred to a yellow stone that was not topaz, but probably chrysolite (chrysoberyl or peridot), it should be borne in mind that topaz is likely not meant here.[20]

Superstition

An English superstition held that topaz cured lunacy.[21]

References

  1. ↑ Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., ISBN 0-471-80580-7
  2. ↑ Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C., eds. (1995). "Topaz". Handbook of Mineralogy (PDF). II (Silica, Silicates). Chantilly, VA, US: Mineralogical Society of America. ISBN 978-0-9622097-1-0. Retrieved December 5, 2011.
  3. ↑ Topaz. Mindat.org. Retrieved on 2011-10-29.
  4. ↑ Topaz. Webmineral.com. Retrieved on 2011-10-29.
  5. 1 2 Karen Hurrell; Mary L. Johnson (15 December 2016). Gemstones: A Complete Color Reference for Precious and Semiprecious Stones of the World. Book Sales. p. 169. ISBN 978-0-7858-3498-4.
  6. ↑ Utah State Gem – Topaz. Pioneer.utah.gov (2010-06-16). Retrieved on 2011-10-29.
  7. ↑ Imperial Topaz Archived 2009-05-13 at the Wayback Machine., Natural History Museum of Los Angeles County
  8. 1 2 Gemstones & Gemology – Topaz, Emporia State University
  9. ↑ State Gem – Texas Blue Topaz. State Gemstone Cut – Lone Star Cut. state.tx.us
  10. ↑ Mystic Topaz, Consumer Information. Farlang.com (2008-10-30). Retrieved on 2011-10-29.
  11. ↑ Renee Newman (7 January 2015). Gem & Jewelry Pocket Guide: A traveler's guide to buying diamonds, colored gems, pearls, gold and platinum jewelry. BookBaby. p. 104. ISBN 978-0-929975-49-8.
  12. ↑ H. Dake, (16 April 2013). The Art of Gem Cutting - Including Cabochons, Faceting, Spheres, Tumbling and Special Techniques. Read Books Limited. p. 105. ISBN 978-1-4474-8480-6.
  13. ↑ "Topaz Guide". Ayana Jewellery. Retrieved November 23, 2016.
  14. ↑ Famous and Notheworthy Topazes Rao Bahadur, A Handbook of Precious Stones, Geological Survey of India
  15. ↑ "Topaz (Blue)". Cape Minerals. Retrieved 7 February 2017.
  16. ↑ Handbook of Texas Online – Mineral Resources and Mining. Tshaonline.org. Retrieved on 2011-10-29.
  17. ↑ Mason, Texas Chamber of Commerce Web site
  18. 1 2 Harper, Douglas. "topaz". Online Etymology Dictionary.
  19. ↑ A Lapidary or History of Gemstones, University of Cambridge, 1652.
  20. ↑ Farrington, Oliver (1903) Gems and Gem Minerals. Chicago. p. 119.
  21. ↑ Pettigrew, Thomas Joseph (1844) On Superstitions Connected with the History and Practice of Medicine and Surgery. Philadelphia E. Barrington and G.D. Haswell. p. 70.
  • Topaz—International Colored Gemstone Association
  • Topaz and other minerals found at Topaz Mountain, Juab County, Utah Geological Survey
  • Wikisource "Topaz". Encyclopædia Britannica. 23 (9th ed.). 1888.
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