Rock & Mineral Encyclopedia

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

Coral Rock

A porous limestone built from the calcium carbonate skeletons of corals and reef organisms, the lithified remains of ancient or modern reefs.

A combustible black sedimentary rock formed from ancient plant matter and burned for centuries as a primary fossil fuel.

A soft, porous limestone made of loosely cemented shell and coral fragments, used as a coastal building stone.

A warm-hued tourmaline blending orange, pink and red tones reminiscent of a sunset sky.

A gentle peach-to-apricot opal, mostly common opal colored by trace iron, prized for its soft warm pastel body.

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

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

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

A lightweight black organic gemstone formed from fossilized wood under pressure, a type of lignite long used in mourning jewelry.

A soft, spongy accumulation of partly decayed plant matter that forms in waterlogged bogs and is the first step toward coal.