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

Soft, lightweight siliceous rock made of fossilized diatom shells, valued as a filter, abrasive, and absorbent.

Fossil shells whose original material has been replaced by opal, preserving ancient marine forms in common or precious opal.

Fossil material whose original substance has been replaced by opal, preserving ancient shapes in common or play-of-color opal.

A limestone packed with visible shells and shell fragments, recording the accumulation of marine invertebrate remains on ancient sea floors.

Calcium-carbonate sedimentary rock packed with visible fossils, recording ancient marine life within an easily scratched, fizzing matrix.

Ancient wood whose organic tissue has been replaced by silica, preserving the grain, rings, and structure of the original tree in stone.

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

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

Fossilized wood in which the original organic structure has been replaced by opal, sometimes showing precious play-of-color.

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