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