Rock & Mineral Encyclopedia

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

Tuff

A light, porous volcanic rock formed from compacted and cemented ash erupted during explosive eruptions.

A fine-grained, intermediate volcanic rock common at subduction-zone volcanoes, between basalt and rhyolite in composition.

A yellow-brown alteration material formed when basaltic volcanic glass reacts with water, common in hydrovolcanic tuffs and pillow lavas.

Rhyolite that has been metamorphosed, recrystallizing its silica-rich volcanic material into a tougher felsic metamorphic rock.

A natural volcanic glass with rich brown and black mahogany-like swirls created by iron oxide inclusions.

A banded rhyolitic volcanic rock with swirling tan, red, and yellow iron-oxide layers prized as a decorative picture stone.

An iron-rich peralkaline rhyolite, a silica-rich volcanic rock named for the Italian island of Pantelleria.

An opaque brown chalcedony whose iron-stained banding mimics deserts, dunes, and distant mountain skylines.

A peralkaline rhyolite, a silica-rich volcanic rock with excess alkalis, named for San Pietro Island's Comende district.

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

A vivid yellow-and-black banded stone from Indonesian volcanic vents, colored by sulfur, arsenic minerals and iron oxides, not true jasper.