Rock & Mineral Encyclopedia

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

Porphyritic Obsidian

Natural volcanic glass speckled with embedded mineral crystals (phenocrysts) such as feldspar or cristobalite that grew before the lava chilled.

Black volcanic glass dotted with small light-colored mineral specks resembling stars scattered across a night sky.

A trade name for sheen obsidian whose swirling, patchy iridescence resembles galaxies and nebulae against deep black glass.

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