igneous
Black-and-white tourmaline granite (likely polished stone)
Tourmaline-bearing granite, chiefly quartz, feldspar, mica, and black schorl tourmaline
AI-generated identification · may be incorrectHardness is about 6–7 (quartz/feldspar; tourmaline 7–7.5); mottled white, gray, and black with a vitreous to greasy polish. Coarse crystalline texture; quartz has no cleavage, feldspar has two cleavages, and tourmaline commonly forms prismatic crystals.
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Physical properties
Hardness is about 6–7 (quartz/feldspar; tourmaline 7–7.5); mottled white, gray, and black with a vitreous to greasy polish. Coarse crystalline texture; quartz has no cleavage, feldspar has two cleavages, and tourmaline commonly forms prismatic crystals.
Formation & geological history
Forms when silica-rich magma slowly cools underground, allowing large interlocking crystals; tourmaline crystallizes from boron-rich late magmatic fluids. Such granites are generally Precambrian to Phanerozoic, depending on the deposit.
Uses & applications
Used as an ornamental, cabochon, or worry stone and occasionally as dimension stone; attractive tourmaline specimens are collected rather than used as gemstones in faceted jewelry.
Geological facts
The black mineral is probably schorl tourmaline, though dark biotite or amphibole is possible from the photograph alone. Granite’s visible interlocking grains distinguish it from many fine-grained volcanic rocks.
Field identification & locations
Look for interlocking white/gray feldspar and quartz with randomly distributed black prismatic grains; a streak test is ineffective, while hardness and crystal form help. Common in granitic pegmatite and granite regions worldwide, including Brazil, the United States, Madagascar, and Europe.
