Rock & Mineral Encyclopedia

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

Dryhead Agate

A banded fortification agate from the remote Dryhead area of south-central Montana, prized for warm concentric rings.

Ancient wood whose organic tissue has been replaced by silica, preserving the grain, rings, and structure of the original tree in stone.

A fine-grained clastic rock of silt-sized grains, intermediate between sandstone and mudstone, with a gritty feel.

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.

A striking color-zoned tourmaline with a pink center and green rind, resembling a slice of watermelon when cut across the crystal.