Rock & Mineral Encyclopedia

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

Lake Huron Agate

Glacially transported banded agates found along Lake Huron's shores, typically small, frosted pebbles with red-orange iron banding.

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

A lightweight black organic gemstone formed from fossilized wood under pressure, a type of lignite long used in mourning jewelry.

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

Boulder opal from the Koroit field in Queensland, famous for intricate ironstone matrix patterns laced with colorful precious opal.

Precious opal that forms in thin veins within brown ironstone boulders, cut with the host rock left as a natural dark backing.

Boulder opal from the Quilpie district of Queensland, Australia, with bright color set in dark ironstone matrix.