Rock & Mineral Encyclopedia

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

Chalk

A soft, white, fine-grained limestone made almost entirely of microscopic marine plankton skeletons.

A banded variety of chalcedony quartz, classically black or black-and-white, long favored for cameos and beads.

A white, porous borate mineral webbed with gray-black veins, widely dyed to imitate turquoise and other stones.

A dark, glassy rock formed when frictional heat from fault movement or impact melts rock along narrow veins.

The sodium end-member of the plagioclase feldspar series, a common white rock-forming mineral and parent of peristerite moonstone.

A soft, fine-grained, porous white limestone made largely of microscopic calcareous plankton skeletons, the rock that forms classic white cliffs.

A spectacularly banded agpaitic nepheline syenite of alternating red eudialyte, black amphibole and white feldspar layers from Ilimaussaq, Greenland.

Precious opal that forms in thin veins within brown ironstone boulders, cut with the host rock left as a natural dark backing.

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

A mica schist containing bladed blue kyanite crystals, a marker of medium- to high-grade metamorphism of aluminous rocks.

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

A richly colored red banded agate prized by lapidaries for its deep crimson-to-brown tones reminiscent of pigeon's blood.