Slate

Hardness: 2.5-4 Mohs scale; Color: typically dark gray, black, or greenish; Luster: dull to slightly silky; Crystal Structure: fine-grained foliated; Cleavage: excellent slaty cleavage; Specific Gravity: 2.7-2.8

Formed through the low-grade regional metamorphism of shale or mudstone. It occurs when fine-grained sedimentary rock is subjected to high pressure and low temperatures, usually associated with plate tectonic collisions and mountain building processes.

Extensively used in construction for roofing shingles, flooring, flagstones, and billiard table tops. Historic industrial use for blackboards and writing slates. High collecting value for fossils sometimes preserved within the layers.

Slate can be split into very thin, flat sheets due to its slaty cleavage. Because it is chemically inert and water-resistant, it has been a premier roofing material for centuries, with some slate roofs lasting over 100 years.

Identify in the field by its distinct ability to split into thin, flat plates and its 'clinking' sound when struck. Commonly found in mountainous regions such as the Appalachian Mountains in the US, Wales in the UK, and parts of Brazil. Collectors should look for signs of foliation and a fine-grained, non-reflective surface.