Rock & Mineral Encyclopedia

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

Seam Agate

Agate that forms in flat cracks or veins of host rock rather than rounded nodules, producing straight, parallel banding.

Black volcanic glass marked with rounded spots and patches that resemble a leopard's coat, caused by spherulitic crystallization.

A chalcedony agate marked by round, concentric ring patterns that resemble eyes when cut and polished.

Limestone built from tiny rounded ooid grains resembling fish roe, formed in warm, agitated shallow seas.

A rare agate that forms naturally with flat polygonal faces and angular geometric shapes rather than the usual rounded nodule.

Rounded nodules of translucent obsidian, named after a Native American legend, that glow smoky brown when held to light.

A silver-white cobalt arsenic sulfide that is a leading ore of cobalt, forming bright metallic cubic and pyritohedral crystals.

A soft pink to crimson hydrated cobalt arsenate, famous as cobalt bloom that signals nearby cobalt ores.

A metallic cobalt-nickel arsenide, an important cobalt ore, named for the Skutterud mines of Norway.

A durable magnesium aluminum oxide gem that occurs in many colors and was long mistaken for ruby.

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

Obsidian with a vivid blue sheen or hue; natural blue obsidian is rare, and intensely uniform blue material is usually manufactured glass.