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

Opal Geoscience Australia

Boulder opal

Crystals, Free Full-Text

e-Rocks Mineral Auctions

Association between opal occurrences and soil type and regolith type

Opal - Wikipedia

Green Jelly Opal (rare color and locality)

Opal - Wikipedia

Opal Meaning, Definitions, History and Folklore Fact Sheet about Opal - AlphaVariable

File:Black opal (Stayish Mine, Wollo Province, Ethiopia) 6 (23226115484).jpg - Wikipedia