Rock & Mineral Encyclopedia

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

Conglomerate

A coarse sedimentary rock of rounded pebbles and gravel cemented in a finer matrix, recording ancient rivers and beaches.

A general term for light-colored, fine-grained volcanic rocks rich in quartz and feldspar, like rhyolite.

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

A fine-grained, silica-rich volcanic rock that is the extrusive equivalent of granite, often pale, banded, or flow-textured.

A fine-grained green phonolitic dike rock rich in nepheline and aegirine, the hypabyssal equivalent of phonolite.

A fine-grained, dark volcanic rock that erupts as fluid lava and forms most of the ocean floor and many lava plateaus.

A fine-grained volcanic rock intermediate between andesite and rhyolite, common at explosive stratovolcanoes.

A fine-grained volcanic rock dominated by alkali feldspar, the extrusive equivalent of syenite.

Rounded nodules of translucent obsidian, named after a Native American legend, that glow smoky brown when held to light.

The fine-grained volcanic equivalent of monzonite, an intermediate lava with nearly equal feldspars and little free quartz.

A conglomerate altered by heat and pressure, often with its rounded pebbles stretched and flattened into elongated lenses.

A hard, fine-grained siliceous rock with a dull porcelain-like texture, intermediate between soft diatomite and dense chert.