The blue or violet color disappears on heating and can be restored by gamma ray bombardment. The natural color may therefore be caused by natural radiation. Gemstones are very fragile due to excellent cleavages and must be cut and handled with great care. [caption id="attachment_182221" align="alignnone" width="402"] By Rock Currier (http://www.mindat.org/photo-217328.html) [CC-BY-3.0], via Wikimedia Commons[/caption]
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Crystallography||Orthorhombic. Crystals equant, thick, tabular or (rarely) prismatic; massive, cleavable.|
|Colors||Colorless, white-gray, bluish, violet, pinkish, reddish, brownish.|
|Luster||Greasy; pearly on cleavage; vitreous in massive varieties.|
|Cleavage||Perfect 1 direction, nearly perfect 1 direction|
|Stone Sizes||Faceted gems are quite unusual, usually small (1-5 carats), but potentially much larger. Gems up to 9 carats have been cut, but cleavage masses could provided larger rough. Faceted gems are usually purplish or pale pink from the Swiss and Canadian localities. PC: 2.86 (pink-blue bicolor, Bancroft).|
|Luminescence||Red color in LW (Germany)|
|Pleochroism||In violet crystals: colorless-pale yellow/ pale violet-rse/violet.|
Optics: α = 1.570; β = 1.575 γ= 1.614.
Biaxial (+), 2V ~43°
Occurrence: A rock-forming mineral, associated with gypsum beds, halite, and limestones. Also occurs in hydrothermal veins, cavities in basalts, and other traprocks.
South Dakota, New Mexico, New Jersey, Texas, Nova Scotia
France, India, Germany, Austria, Poland.
Faraday Mine, Bancroft, Ontario, Canada: large purplish masses, some facetable.
Simplon Tunnel, Switzerland: pale purple cleavages, facetable.
Mexico: large blue masses, very lovely color.
Volpino, Italy: a white-gray, marble like textured material known as vulpinite and locally used as a decorative stone and made into cabochons.
Comments: The blue or violet color disappears on heating and can be restored by gamma ray bombardment. The natural color may therefore be caused by natural radiation. Gemstones are very fragile due to excellent cleavages and must be cut and handled with great care.
Names: Greek without water, in allusion to composition.