igneous
Quartz-rich pegmatite (likely tourmaline-bearing)
Granitic pegmatite; primarily quartz (SiO₂), with feldspar and dark tourmaline/biotite
AI-generated identification · may be incorrectHardness ~7 for quartz; coarse, interlocking crystals with vitreous luster, gray-white quartz, pale feldspar, and black/brown accessory minerals. No consistent cleavage in quartz; feldspar shows two cleavages.
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Physical properties
Hardness ~7 for quartz; coarse, interlocking crystals with vitreous luster, gray-white quartz, pale feldspar, and black/brown accessory minerals. No consistent cleavage in quartz; feldspar shows two cleavages.
Formation & geological history
Forms from volatile-rich, silica-saturated magma cooling slowly in dikes or veins, producing exceptionally coarse crystals; commonly Precambrian to Mesozoic, depending on locality.
Uses & applications
Used as a decorative or landscaping stone and as a source of quartz, feldspar, mica, and occasionally gem tourmaline or beryl; chiefly a collector specimen when attractive.
Geological facts
The dark prismatic material may be schorl tourmaline, while rusty areas are iron-oxide weathering. Pegmatites can host rare elements such as lithium, tantalum, and cesium.
Field identification & locations
Identify by very coarse crystal size, glassy gray quartz, blocky feldspar, and possible black tourmaline; test quartz with a steel file or glass (quartz scratches glass). Common in granitic terrains worldwide, including Brazil, Madagascar, the United States, and Scandinavia.
