Rock & Mineral Encyclopedia

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

Brecciated Agate

Agate that was shattered and naturally re-cemented by silica, creating a mosaic of angular fragments in a quartz matrix.

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

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

A nodular rhyolite geode-like ball whose plain exterior hides a star-shaped agate or chalcedony core when cut.

A rare zirconium-bearing garnet that crystallizes in carbonatites and alkaline rocks, first described from Magnet Cove, Arkansas.

An opaque, snow-white variety of quartz whose milky color comes from countless tiny gas and fluid inclusions.

Iron-stained sandstone whose red color comes from hematite coatings, formed in oxidizing desert, river, and coastal environments.

An orange-to-rust colored variety of natural volcanic glass whose warm tone comes from iron oxide staining within the obsidian.

An extremely common calcium carbonate mineral that comes in nearly every color and shows strong double refraction in clear crystals.

A translucent, soft blue variety of microcrystalline quartz whose color comes from light scattering through its fine structure.

A chromium-rich pyrope garnet whose intense blood-red color comes from chromium, often mined from ant hills and kimberlite weathering.

A deep blue beryl whose intense color comes from radiation-induced color centers and tends to fade in light, named after the Maxixe mine in Brazil.