Rock & Mineral Encyclopedia

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

Sapropel

A soft, dark, organic-rich mud deposited in stagnant, oxygen-poor water, a key precursor to oil and gas source rocks.

A dark, organic-rich shale loaded with kerogen and bitumen that can yield oil and gas, often finely laminated and combustible.

Dark, organic-rich fine-grained sedimentary rock formed in oxygen-poor waters, often a source rock for oil and gas.

Ancient wood whose organic tissue has been replaced by silica, preserving the grain, rings, and structure of the original tree in stone.

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

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.

A fine-grained sedimentary rock rich in solid organic matter (kerogen) that yields oil and gas when heated.

An organic gem formed inside mollusks from layered nacre, prized for its iridescent luster and classic elegance.

A patterned jasper whose scaly, net-like markings recall reptile skin, popular with lapidaries for its organic camouflage look.

Fossilized wood in which the original organic structure has been replaced by opal, sometimes showing precious play-of-color.

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