Slate

Physical Properties

Hardness: 2.5–4 on Mohs scale; Color: Dark grey, charcoal; Luster: Dull to satiny; Crystal Structure: Microcrystalline; Cleavage: Perfect slaty cleavage (splits into thin, flat sheets); Specific Gravity: 2.7–2.8

Formation & Geological History

Formed from the low-grade metamorphism of shale or mudstone under relatively low pressure and temperature conditions. It typically dates back to various geological eras depending on regional tectonic history, often found in ancient mountain belts.

Uses & Applications

Used for roofing tiles, flooring, electrical panels, chalkboards, billiard table tops, and decorative garden stones.

Geological Facts

Slate is unique because its minerals (mostly micas) align so perfectly during metamorphism that it can be split into sheets as thin as a few millimeters. Some slate specimens contain fossils that survived the metamorphic process.

Field Identification & Locations

Identify in the field by its distinct ability to ring when struck and its tendency to break into flat, sharp-edged plates. It is commonly found in regions like Wales, the Appalachian Mountains in the US, and Brazil.

Identified on: 4/20/2026

Mode: Standard