Opal: Mineral information, data and localities.

Although it is still (2007) regarded as a mineral species for historical reasons, opal is not a true mineral in the accepted sense of the word as it is

Onyx Opal: Mineral information, data and localities.

Bone Opal: Mineral information, data and localities.

Opal: The mineral Opal (black, white, fire) information and pictures

Opal Mineral Information photos and Facts, Manganese Ore

Opal: Mineral information, data and localities.

Opal, Properties, Types & Uses

Opal - Wikipedia

Precious opal (Mezezo area, Ethiopia) 6, Precious opal from…

Common Opal: Mineral information, data and localities.

Opal : Properties, Formation, Uses and Deposits » Geology Science

Meet a Nevada Mineral: Opal - Nevada Mining Association

Opal: The mineral Opal (black, white, fire) information and pictures