metamorphic

Slate

Slate; fine-grained metamorphosed shale, commonly composed of quartz, illite/muscovite, and chlorite

AI-generated identification · may be incorrect

Usually dark gray to black, fine-grained, dull to silky luster, with excellent slaty cleavage and very low porosity; hardness about 3–4 Mohs. The specimen shows closely spaced cleavage/fissile layers and faint foliation.

Identified September 22, 2026More metamorphic →

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Physical properties

Formation & geological history

Forms when shale or mudstone undergoes low-grade regional metamorphism, typically during mountain building; its age depends on the source formation, commonly Paleozoic to Precambrian. It forms under directed pressure and relatively low temperatures.

Uses & applications

Used for roofing, floor and wall tiles, paving, billiard tables, writing boards, and decorative stone; attractive pieces may be sold as teaching or lapidary specimens.

Geological facts

Slate splits into thin, relatively smooth sheets because aligned clay minerals recrystallize during metamorphism. It grades metamorphically toward phyllite as mica crystals grow larger.

Field identification & locations

Identify it by its uniform fine texture, gray-black color, and ability to split into flat sheets along cleavage; unlike shale, it is harder and cleavage surfaces are typically more planar. Common in metamorphic belts worldwide, including Wales, the northeastern United States, Spain, and Brazil.