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

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

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.

Ancient wood whose organic tissue has been replaced by silica, preserving the grain, rings, and structure of the original tree in stone.

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

Fossilized tree resin, warm and lightweight, sometimes preserving ancient insects and plant matter inside.

A dark green-and-black stromatolite jasper patterned with swirling orbs, formed from fossilized ancient microbial colonies.

Fossilized wood in which the original organic structure has been replaced by opal, sometimes showing precious play-of-color.