metamorphic
Slate
Slate; fine-grained metamorphosed shale, chiefly quartz, mica, and clay minerals
AI-generated identification · may be incorrectTypically dark gray, brown, purple, or green with a dull to slightly silky luster and very fine grains. Hardness is about 3–4 Mohs; strong slaty cleavage allows it to split into thin, flat sheets, as visible here.
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Physical properties
Typically dark gray, brown, purple, or green with a dull to slightly silky luster and very fine grains. Hardness is about 3–4 Mohs; strong slaty cleavage allows it to split into thin, flat sheets, as visible here.
Formation & geological history
Forms when shale or mudstone undergoes low-grade regional metamorphism, commonly during mountain building. Its age depends on the source sediment and metamorphic event; slate occurs from Precambrian to recent geological settings.
Uses & applications
Used for roofing, floor and wall tiles, paving, blackboards, billiard-table surfaces, and decorative stone. Common material has modest value, usually about $5–$30 per hand specimen; attractive architectural or collectible pieces may cost $50–$200.
Geological facts
Slate is distinguished from shale by its metamorphic cleavage and stronger, more regular splitting. It commonly contains tiny aligned mica grains that produce its planar fabric.
Field identification & locations
Common in metamorphic belts worldwide, including Wales, Vermont, Pennsylvania, Spain, Brazil, and China. Identify it by its fine texture, planar cleavage, relatively hard surfaces, and lack of visible sand-sized grains; mica-rich bands or quartz veins may occur.
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