Rock & Mineral Encyclopedia

Search and identify 1,000+ rocks, minerals, crystals, and gemstones — with properties, formation, colors, hardness, and how to tell them apart.

Argillite

Hardened, fine-grained mudrock intermediate between shale and slate, dense and non-fissile, often carved into ornaments.

Dark, organic-rich fine-grained sedimentary rock formed in oxygen-poor waters, often a source rock for oil and gas.

A dark, organic-rich shale loaded with kerogen and bitumen that can yield oil and gas, often finely laminated and combustible.

A fine-grained sedimentary rock of compacted clay and silt that, unlike shale, breaks in blocks rather than thin layers.

The smoky brown to gray variety of quartz, colored by natural irradiation, valued as both a gemstone and crystal specimen.

Translucent smoky-gray obsidian that transmits a hazy light, intermediate between clear and fully black volcanic glass.

Opal with a dark grey body tone sitting between black and light opal, giving play-of-color rich contrast at an accessible price.

A dark gray-to-black feldspar variety of moonstone that shows blue and white adularescent flash against a smoky body.

A plagioclase feldspar famous for labradorescence, a dramatic flash of iridescent blue, green, and gold across a dark gray stone.

A dark gray copper sulfide that is one of the richest copper ores, prized by collectors when found as rare sharp crystals.

A dark, foliated schist rich in graphite that leaves a grey-black mark and forms from metamorphosed carbon-rich sediments.