Oregon Opal Identification Guide
Identify Oregon opal, a translucent blue-green to amber hydrated silica from Oregon, including its tendency to craze.
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What Oregon Opal Looks Like
Oregon opal is hydrated silica (SiO2.nH2O) from Oregon, USA, typically a translucent to transparent stone in soft blue-green, teal, amber, colorless, or pinkish tones. Most is common opal with a glowing waterlike clarity; some shows a faint blue Tyndall sheen, and rarer pieces flash play-of-color. It is known for its watery translucency and a tendency to craze (develop fine cracks) as it dries.
- Color: blue-green, teal, amber, colorless, pink
- Luster: vitreous to resinous
- Transparency: transparent to translucent
- Form: massive, in seams and nodules within volcanic host rock
Step-by-Step Field ID Checklist
- Backlight it. A glowing, watery translucency with a soft body color is typical.
- Look for a blue sheen. Many Oregon opals show a faint bluish Tyndall glow in reflected light.
- Check for crazing. Fine internal or surface cracks confirm high water content opal.
- Test hardness. It scratches with a steel knife (softer than quartz).
- Feel the weight. It is notably light for its size.
Key Diagnostic Tests
- Hardness: Mohs 5.5-6.5.
- Streak: white.
- Fracture: conchoidal; no cleavage (amorphous).
- Density: low, about 1.9-2.2 g/cm3.
- Acid: no reaction.
- UV: may fluoresce greenish (uranium-bearing Oregon opal can glow under UV).
Common Look-Alikes
- Chalcedony/blue agate: harder (Mohs 7), denser, no crazing; resists a knife where opal scratches.
- Blue glass: look for round bubbles, higher density, and no natural crazing.
- Larimar: opaque blue pectolite, harder texture, with white veining, not a glowing translucent silica.
- Aquamarine: crystalline beryl, hardness 7.5-8, far harder and denser with hexagonal habit.
Where It Is Found
Oregon opal comes mainly from the Opal Butte deposit in Morrow County and other volcanic localities in eastern Oregon. It forms in cavities and seams within rhyolitic and other volcanic rocks where silica-rich groundwater deposited the opal at low temperatures.
Frequently asked questions
How can you tell if Oregon opal is real?
Real Oregon opal is soft (Mohs 5.5-6.5), light in weight, glows with watery translucency when backlit, and often shows faint blue sheen or fine crazing cracks. Harder, heavier, bubble-filled material is glass or chalcedony.
Why does Oregon opal crack or craze?
Oregon opal holds water in its structure. As it loses moisture over time, internal stress can produce fine crazing cracks, which is a known trait of this and other hydrophane-type opals.
Does Oregon opal show fire?
Most Oregon opal is common opal with only a soft body color or faint blue sheen, but rarer pieces do flash spectral play-of-color.
What does Oregon opal look like?
It looks like a glowing, watery, translucent stone in blue-green, teal, amber, or colorless tones, sometimes with a faint blue sheen.