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Rock guide

Opal

gem · common

Quick facts

Formula
SiO₂·nH₂O
Hardness
5.5–6.5
Luster
Vitreous
Colors
Various

Opal is a hydrated amorphous form of silica, known for its unique play of color. It is found in various locations worldwide.

Look for vibrant play of color and lack of crystalline structure.

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