Nevada Opal Identification Guide
A guide to identifying Nevada opal, including Virgin Valley black opal, by its silica body, play-of-color, low density, and tendency to craze.
Read the full Nevada Opal encyclopedia entry →
What Nevada Opal Looks Like
Nevada opal is hydrated silica (SiO2·nH2O) from Nevada, most famously the Virgin Valley black and crystal opal — often opalized wood and casts with vivid play-of-color. Much Nevada opal is hydrophane, meaning it absorbs water and can change appearance (and may craze/crack as it dries).
- Color: dark to black, grey, or colorless body; flashes of red, green, blue, orange play-of-color
- Luster: vitreous to waxy/resinous
- Transparency: transparent (crystal opal) to opaque (black/wood opal)
- Habit: amorphous; seam fill, nodules, and replaced wood preserving grain
- Play-of-color: the diagnostic rainbow flash that shifts as the stone is turned
Step-by-Step Field ID Checklist
- Look for play-of-color — flashes that move and change with viewing angle (separates precious from common opal).
- Confirm low heft — opal is light for its size.
- Note any wood grain or cast structure (common in Virgin Valley material).
- Test hardness — it is scratched by quartz (5.5-6.5).
- Watch for crazing (fine surface cracks) typical of unstable Nevada opal as it dries.
Key Diagnostic Tests
- Mohs hardness: 5.5-6.5; softer than quartz/chalcedony.
- Streak: white.
- Cleavage/fracture: none; conchoidal fracture.
- Density: low, SG ~1.9-2.2.
- Acid: no reaction.
- Hydrophane behavior: may stick to a wet tongue or absorb water and temporarily change clarity — a useful field clue for this material.
Common Look-Alikes and How to Tell Them Apart
- Australian opal: Australian black opal is more stable (less crazing) and not hydrophane; Nevada opal often crazes and may be hydrophane.
- Common opal/opalite: lacks play-of-color entirely; Nevada precious opal flashes color.
- Labradorite/moonstone: show schiller/adularescence but are far harder (6-6.5+ feldspar), denser, and crystalline with cleavage; opal is amorphous with no cleavage.
- Glass/imitation opal (e.g., Slocum stone): look for regular bubble patterns, foil-like flakes, and higher stability; natural opal's color play is irregular and three-dimensional.
Where It Is Found
Nevada opal centers on the Virgin Valley, Humboldt County, Nevada, famous for black opal and opalized wood, with additional common and precious opal occurrences across Nevada's volcanic terrains.
Frequently asked questions
How can you tell if it's real Nevada opal?
Real Nevada opal is amorphous silica with a hardness of 5.5-6.5, low density (1.9-2.2), no cleavage, and irregular three-dimensional play-of-color in precious material. Much of it is hydrophane and prone to crazing, unlike glass imitations with regular bubbles.
Why does Nevada opal crack or craze?
A lot of Nevada opal, especially Virgin Valley material, is hydrophane and contains water that escapes as it dries, causing fine surface cracks called crazing. Stable storage and slow drying help reduce it.
What is Virgin Valley opal?
Virgin Valley, in Humboldt County, Nevada, is famous for black opal and opalized wood with vivid play-of-color; much of it is hydrophane and can be unstable.
Nevada opal vs Australian opal — what's the difference?
Australian black opal is generally more stable and not hydrophane, while Nevada opal is often hydrophane and more prone to crazing, though both can show stunning play-of-color.