Rock & Mineral Encyclopedia

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

Orthoclase

A common rock-forming potassium feldspar, the Mohs hardness reference at 6, found in granites and used in ceramics and glassmaking.

A durable magnesium aluminum oxide gem that occurs in many colors and was long mistaken for ruby.

The classic gem opal that flashes shifting spectral colors, defined by the diffraction effect known as play-of-color.

The rarest and most valuable opal, with a dark body tone that makes its flashing rainbow play-of-color blaze brilliantly.

The most common precious opal, with a pale milky body that shows softer pastel flashes of play-of-color throughout.

Opal from Australia, the world's leading source of precious opal, ranging from white and crystal to prized black and boulder types.

A nodular rhyolite geode-like ball whose plain exterior hides a star-shaped agate or chalcedony core when cut.