Rock & Mineral Encyclopedia

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

Fossiliferous Limestone

Calcium-carbonate sedimentary rock packed with visible fossils, recording ancient marine life within an easily scratched, fizzing matrix.

Hardened, fine-grained mudrock intermediate between shale and slate, dense and non-fissile, often carved into ornaments.

A fossiliferous jasper packed with spiral snail shells, technically a silicified gastropod limestone from Wyoming.

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

A brown fossiliferous chalcedony packed with spiral freshwater snail shells, technically agatized fossil rock from Wyoming.

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

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

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

Fossil material whose original substance has been replaced by opal, preserving ancient shapes in common or play-of-color opal.

A translucent chalcedony decorated with branching, fern-like manganese or iron oxide inclusions resembling tiny plants.