Rock & Mineral Encyclopedia

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

Migmatite

A 'mixed rock' showing swirling light and dark bands, formed where high-grade metamorphism causes rock to begin partially melting.

Opal without play-of-color, valued for solid body hues; also called potch, it occurs in a wide range of colors worldwide.

A soft, earthy sedimentary rock made of a mixture of calcium carbonate and clay, intermediate between limestone and mudstone.

A graded sedimentary deposit laid down by underwater turbidity currents, recording avalanches of sediment cascading down submarine slopes.

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

An earthy aluminum-rich residual rock and the world's principal ore of aluminum, often showing distinctive pea-like pisolites.

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

The highest-rank coal, a hard, lustrous black rock that burns cleanly with little smoke and high heat output.

A dense, black, mid-rank coal with high energy content, widely used for power generation and to make coke for steelmaking.