Slate

Slate is a fine-grained, foliated metamorphic rock that forms primarily from the low-grade metamorphism of shale or mudstone. The most distinctive feature of slate is slaty cleavage, a planar splitting that allows the rock to be shaved or split into thin, durable sheets. This cleavage results from the parallel alignment of platy minerals such as chlorite, muscovite (sericite), and biotite during metamorphism under directed pressure in crustal deformation zones.

Slate is typically very fine-grained and dense, with a dull to metallic luster. It commonly occurs in gray, green, or bluish-black colors, though it can show other hues depending on mineral content and impurities. The grains are too small to see with the naked eye, and the rock is relatively hard and resistant to weathering, making it suitable for outdoor use in roofing, flooring, and paving.

Historically, slate has been quarried for roofing shingles and blackboards; modern uses include roofing slates, floor tiles, countertops, and dimension stone for monuments or construction.

Geologically, slate forms at low metamorphic grades from shale or mudstone under directed pressure during mountain-building events. It is distinguished from higher-grade metamorphic rocks like phyllite and schist by its clear slaty cleavage and lack of coarse foliation.

Major sources include parts of the British Isles (notably Wales and Cornwall), Spain, and various regions in North America and elsewhere.