Rock & Mineral Encyclopedia

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

Lignite

The lowest rank of coal, a soft brown carbon-rich rock formed from compacted peat, used mainly for electricity generation.

Limestone built from tiny rounded ooid grains resembling fish roe, formed in warm, agitated shallow seas.

A soft pink common opal from the Peruvian Andes, prized for its opaque rosy color rather than play-of-color.

A limestone made of tiny spherical ooids, resembling fish roe, formed in warm, agitated shallow seas.

A banded, metamorphosed iron formation in which bright red jasper alternates with silvery hematite or magnetite layers.

Ancient chemically deposited rock of alternating iron-oxide and silica bands recording Earth's early oxygenation and a major iron ore source.