Opal
“Fire in a stone”
Mineral
Opal is a hydrated amorphous form of silica distinguished by its stunning play of color caused by the diffraction of light through microscopic silica spheres arranged in a regular pattern. Unlike most gems, opal contains up to 20% water trapped within its structure.
Composition
hydrated silicon dioxide (SiO₂·nH₂O)
Where Found
Coober Pedy (Australia — produces ~90% of gem opals), Ethiopia, Mexico, Nevada (USA)
Mohs Hardness
5.5-6.5
Fun Fact
Opal has been found on Mars by NASA rovers, suggesting that liquid water once existed on the Martian surface.
Test Your Knowledge
Why must opal gemstones be stored and handled with special care?
Related Specimens
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