metamorphic

Slate

Slate; fine-grained foliated metamorphic rock, chiefly quartz, mica, and clay minerals

AI-generated identification · may be incorrect

Typically dark gray to black, fine-grained, dull to silky luster, with strong slaty cleavage allowing thin, flat sheets; hardness about 3–4 Mohs and specific gravity about 2.7–2.8. The specimen shows pronounced planar foliation and rusty iron-oxide staining.

Identified September 13, 2026More metamorphic →

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Physical properties

Formation & geological history

Forms when shale or mudstone undergoes low-grade regional metamorphism, commonly during mountain building; its exact geological age depends on the source formation. The layered texture reflects aligned microscopic mica and clay minerals.

Uses & applications

Used for roofing, floor and wall tiles, paving, writing boards, landscaping, and decorative slabs; attractive specimens are collected as representative metamorphic rocks.

Geological facts

Slate is the lowest-grade metamorphic equivalent of shale and commonly preserves original bedding intersected by cleavage. It splits more cleanly than shale because metamorphic minerals have been realigned.

Field identification & locations

Identify it by its very fine grain, flat cleavage, hard durable surfaces, and inability to crumble like shale; common sources include Wales, Spain, Vermont, Pennsylvania, and Brazil. Rust-colored areas are likely weathered iron-bearing minerals.