metamorphic

Slate

Slate; predominantly fine-grained phyllosilicates (muscovite/illite, chlorite) with quartz and possible iron oxides

AI-generated identification · may be incorrect

Typically dark gray, brown, purple, or green; dull to silky luster; very fine-grained with strong slaty cleavage and splintery, planar fracture. Hardness is about 2.5–4 Mohs; specific gravity approximately 2.7–2.8.

Identified September 18, 2026More metamorphic →

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Physical properties

Formation & geological history

Forms when shale or mudstone undergoes low-grade regional metamorphism, realigning clay minerals into cleavage planes. Most examples formed during Paleozoic–Precambrian mountain-building events, though age varies by locality.

Uses & applications

Used for roofing, floor and wall tiles, paving, hearths, blackboards, landscaping, and decorative stone; attractive colored pieces may be collected or fashioned into ornaments.

Geological facts

Slate is the lowest-grade metamorphic equivalent of shale and commonly progresses toward phyllite with increasing heat and pressure. Its cleavage can split rock into thin, durable sheets.

Field identification & locations

Identify it by fine grain, planar splitting, and lack of visible large crystals; it is commonly found in metamorphic belts and former shale terrains worldwide, including Wales, Vermont, Spain, and China.