Mineral Species: Quartz var. Opal 

Chemical Formula: SiO2

Chemical Name: Silicon Dioxide

Mineral Class: Tectosilicate

Mineral Notes: Opal is amorphous silicon dioxide with a significant amount of water retained in the crystal structure. This can reach as high as 20%. The mineral usually forms as a gel which is deposited at relatively low temperature and may occur in the fissures of almost any kind of rock, including limonite, sandstone and basalt. It subsequently loses water and hardens to form opal.  Common opal is truly amorphous, whereas precious opal does have a structural element. The regular array of atoms in the lattice causes diffraction, producing the colour play characterisitc of the mineral. Opal ranges from clear through white, gray, red, yellow, green, shore, blue, magenta, brown, and black. Of these hues, red and black are the most rare and prized, whereas white and green are the most common.

Description: Two opalized cavities in a matrix of brown iron-stone matrix. Both show opalescence. The larger of the two cavities also contains some very nicely formed Quartz crystals which are lustrous and quite gemmy.

Location: Coober Pedy, Southern Australia, Australia.