Rock & Mineral Encyclopedia

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

Septarian Concretion

A rounded sedimentary nodule cracked internally and filled with veins of yellow calcite, prized for its striking dragon-skin patterning.

A hard, dark variety of chert that knaps into razor-sharp edges and sparks against steel, central to Stone Age technology.

An opaque grey-brown common opal forming nodules and concretions, historically called liver opal for its dull brownish color.