skiffern

Skiffern, or slate, is a fine-grained foliated metamorphic rock derived from clay- or mudstone (shale) through low-grade regional metamorphism. The process aligns platy minerals such as mica, producing slaty cleavage, a property that allows slate to be split into thin, durable sheets. Color varies from gray to green, purple, or black, depending on mineral content and impurities. Slate is relatively hard and dense, with low porosity and good thermal and chemical stability, making it suitable for outdoor and architectural use.

Formation and composition: It forms from sedimentary rocks under pressure and temperature conditions insufficient to form higher-grade schists. The slaty cleavage is what gives slate its characteristic ability to be split into smooth, flat sheets.

Uses: The best-known use is roofing slate, where thin slates cover roofs for waterproofing and longevity. It is also used for floor and wall tiles, cemetery and building veneers, countertops in some cases, and historically for blackboards and billiard-table surfaces.

Distribution and production: Slate occurs in many parts of the world wherever low-grade metamorphism of pelitic sediments has occurred. Traditional slate regions include parts of the British Isles, Spain, and Italy, with extensive modern quarries in several countries.

Differences from shale: Slate is metamorphosed shale with foliation and well-developed cleavage, whereas shale is an unmetamorphosed sedimentary rock that breaks along fracture planes rather than cleanly splitting into sheets.