Opal
Precious opal displays a spectacular color-play phenomenon called opalescence — brilliant flashes of spectral colors caused by diffraction of light through silica sphere arrays. Spencer, Idaho has the only significant precious opal deposit in the US open to fee digging. Nevada's Virgin Valley produces spectacular black and crystal opals, some found still in their host wood (replacing ancient logs).
Physical Properties
Chemical Formula
SiO₂·nH₂O (hydrated silica)
Hardness (Mohs)
5.5–6.5
Color
White, black, fire (orange-red), crystal — all with play-of-color
Luster
Waxy to vitreous
Crystal System
None (amorphous)
Streak
White
Cleavage
None
Specific Gravity
1.9–2.3
🔍 Field Identification Tips
Play-of-color (shifting rainbow colors when you rotate the stone) is unmistakable for precious opal. Common opal has no play-of-color. Much lighter than glass. Somewhat soft — scratched by a knife. Handle carefully — opals can crack if dried too quickly (crazing).
Varieties & Forms
📍 Where to Find Opal in the US
Uses & Applications
Jewelry (October birthstone), highly prized collectors specimens, cabochons.
💰 Collector Value Guide
Common opal: $1–$10. Small play-of-color pieces: $10–$100. Quality cabochons: $50–$1,000. Exceptional black opal: $10,000+.
Find Opal With Our Tools
Use the We Lick Rocks app to identify specimens, find dig sites, and connect with local rockhounds.