Opal | ‪#‎Geology

Formula:SiO2 · nH2O
Colour:Colorless, white, various
Hardness:5½ - 6½

Although it is still (2007) regarded as a valid mineral species for historical reasons, Opal is not a true mineral in the accepted sense of the word as it is either composed of Cristobalite and/or Tridymite or composed of amorphous silica.

From Yowah, Queensland, Australia
specimen from the Willems Miner Collection

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