Fire opal (opal)
gem · uncommon
Quick facts
- Formula
- SiO2·nH2O
- Hardness
- 5.5–6.5
- Luster
- Vitreous to subvitreous, often resinous
- Colors
- Colorless to yellow, orange, red, honey; may show play-of-color in some pieces
Opal is a hydrous, amorphous form of silica deposited at low temperatures, commonly in volcanic or sedimentary environments. Fire opal refers to opal with warm body color (yellow to red), which may or may not show play-of-color.
Look for a jelly-like, resinous interior without fibrous or banded chalcedony texture; lower hardness than quartz and possible flashes of play-of-color distinguish it from carnelian.
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