13 Mayıs 2013 Pazartesi

US Virgin Islands Territorial Gemstone

US Virgin Islands Territorial Gemstone


Prehnite is a translucent mineral that can be colorless to gray to yellow, yellow-green, apple green, mint green, light brown, blue, or white. It often occurs in a shade of green unique to prehnite. It is a hydrated silicate of calcium and aluminum.
Although the U.S. Virgin Islands do not have an official gemstone, prehnite represents the territory's geology and economy well, as the mineral occurs on all three islands. Prehnite occurs as spongy crystals in association with the mineral wairakite in the Judith Fancy Formation on St. Croix, in association with pumpellyite on St. John, and at Nadir on St. Thomas.
Prehnite is the first mineral to be named for an individual, and also the first to be named and described from South Africa, a country that today enjoys a thriving gemstone industry. Colonel Hendrik von Prehn (1733-1785), governor of the Dutch colony at the Cape of Good Hope from 1768 to 1780 and also a mineralogist, was the first to discover the mineral. He took it back to Europe, and the stone was posthumously named for him in 1788.
Prehnite is a secondary mineral in volcanic rocks rich in iron and magnesium. It occurs at sites of low-grade metamorphic or hydrothermal activity, in veins and cavities of basaltic and igneous rock. It forms rough, thick crusts of crystals over laumonite crystals. Prehnite is revealed when the laumonite is washed away.
  • South Africa: Cape of Good Hope, Cape Province
  • Namibia: Copper Valley, Brandberg Mountains
  • France: La Combe-de-la-Selle, near Bourg d’Oisans, Is¨¨re
  • Austria: Habachtal, Salzburg, and Ankogel
  • Germany: Radauthal, near Harzburg, Harz Mountains
  • Italy: Campitello, Val di Fassa, Trentino-Alto Adige
  • Switzerland: Comperio, Graub¨¹nden, and St. Gotthard, Ticino
  • United Kingdom: Scotland
  • United States: Paterson, Passaic County, New Jersey; Bergen Hill, Hudson County, New Jersey; Fairfax quarry, Centreville, Fairfax County, Virginia; Roncari quarry, East Granby, Hartford County, Connecticut; Hampden quarry, West Springfield, Hampden County, Massachusetts; Upper Peninsula/Lake Superior region, Michigan
  • Canada: Jeffrey mine, Asbestos, Quebec
  • India: Khandivali quarry, Bombay, Maharashtra, India
  • Australia: Coonabarabran, Warrumbungle Ranges, and Emu quarry, Prospect Hill, near Sydney, New South Wales
  • Antarctica: Rennick Glacier, North Victoria Land
Appreciated as an interesting collection mineral, prehnite is becoming more common as a gemstone. Prehnite a rare gem until about the 1980s, when large deposits were found in Australia. The stone is now more available and consequently more affordable. It may still be somewhat difficult to find in local jewelry stores, however. The stone has traditionally been cut as a cabochon, carved, or cut and polished as an ornament. More recently, faceted prehnite has become popular in jewelry, as have prehnite beads. Higher quality faceted stones and clear mint green specimens are the most prized and command the highest prices.
Prehnite is said to encourage composure, forgiveness, and acceptance. Some consider it a stone of unconditional love, and is believed to be an excellent stone for meditation. Some people also believe that prehnite helps relieve nightmares and many phobias.
State Gemstone
State Gemstone
Unusual Prehnite Shapes
State Gemstone
Prehnite Close Up
State Gemstone
Isolated Prehnite
Group: Silicates 
Chemical Formula: Ca2Al(AlSi3O10)(OH)2
Crystal Structure: Orthorhombic-Pyramidal (mm2)
Hardness (Mohs): 6-6.5
Color: Colorless to gray to yellow, yellow-green or white
Transparency: Translucent
Luster: Vitreous, pearly
Birefringence: ¦Ä = 0.021 - 0.033
Refractive Index: n¦Á = 1.611 - 1.632 n¦Â = 1.615 - 1.642 n¦Ã = 1.632 - 1.665
Density: 2.8-2.95
Streak: White
Cleavage: Distinct/good,
good on {001}, poor on {110}
Fracture: Irregular/uneven

Some data courtesy of the Mineralogical Society of America
Author: World Trade Press

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