Rock & Mineral Encyclopedia

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

Shell Opal

Fossil shells whose original material has been replaced by opal, preserving ancient marine forms in common or precious opal.

Coarse-grained gabbro that has been metamorphosed, partly recrystallizing into amphibole, plagioclase, and other metamorphic minerals.

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

Fossil bone in which opal has replaced the original tissue, sometimes showing play-of-color, a rare collector fossil.

Fossil material whose original substance has been replaced by opal, preserving ancient shapes in common or play-of-color opal.