Datolite is a rather soft gemstone if wear is considered. The nodules come in very attractive colors. Faceted stones are extremely brilliant, though their dispersion (fire) is low. Most faceted gems are colorless, pale yellowish or pale green.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Crystallography||Monoclinic. Crystals prismatic or stubby; massive, granular.|
|Colors||Colorless, white, pale yellow, green, also pink, reddish, and brownish due to impurities; massive varieties can be white to orange-brown or pink.|
|Cleavage||None: fracture uneven to conchoidal; brittle.|
|Stone Sizes||Brown or white massive material will cut cabochons up to several ounces. The colors in the Michigan material are due to copper staining. Cabochons are seldom seen in collections—collectors prefer to polish the faces of sliced-open nodules. These can be up to about 6 inches in diameter. The best faceting material comes from Massachusetts, with fine pale green material from New Jersey. The largest gems cut from this are in the 5-carat range. Larger stones are very rare. Smithsonian Institution (Washington, D.C): 5.4 and 5.0 (colorless, Massachusetts). Harvard University: 13.21 (flawless triangle, Massachusetts). National Museums of Canada (Ottawa, Ontario): 0.45(colorless, Canada).|
|Luminescence||Blue in SW (attributed to the presence of Eu).|
Optics: a = 1.622-1.626: β= 1.649-1.658; γ=1.666 1.670.
Biaxial ( – ), 2V= 75°.
Occurrence: A secondary mineral in basic igneous rocks and traprocks.
Springfield, Massachusetts; Lane’s Quarry, Westfield, Massachusetts; Paterson, New Jersey (and other localities in the state).
Faraday Mine, Ontario, Canada: colorless material. Tyrol, Austria; Habach. Austria; Cornwall, England Lake Superior Copper district. Michigan: nodules of massive datolite.
Comments: Datolite is a rather soft gemstone if wear is considered. The nodules come in very attractive colors. Faceted stones are extremely brilliant, though their dispersion (fire) is low. Most faceted gems are colorless, pale yellowish or pale green.
Name: From a Greek word meaning to divide because of the granular nature of the massive variety.