Spencer Opal Identification Guide
Identify Spencer Opal, the thin-layered Idaho precious opal, by its play-of-color, hydrophane behavior, host rhyolite, and triplet construction.
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What Spencer Opal Looks Like
Spencer Opal is a precious opal from Spencer, Idaho, famous for vivid play-of-color in pinfire and broad flash patterns. The natural seams are extremely thin, so most Spencer opal is cut as triplets (a thin opal layer between a dark backing and a clear quartz or glass cap). Body color ranges from milky white to gray, with flashes of green, blue, orange and red.
- Luster: vitreous to waxy/resinous
- Transparency: translucent to opaque (the precious layer is translucent)
- Habit: amorphous (non-crystalline) silica; forms as thin layers and seams in volcanic host rock
Step-by-Step Field-ID Checklist
- Look for play-of-color. Move the stone under light — true opal flashes spectral colors that move and change, unlike a fixed body tint.
- Inspect the edge of a cut stone. A flat, dead-straight color boundary or a glass-clear dome over a thin colored band reveals a triplet or doublet — typical of Spencer material.
- Note the pattern. Spencer opal often shows fine pinfire (tiny dots) and broad-flash play in greens and blues.
- Check the host. Rough Spencer opal sits in seams within rhyolitic volcanic rock with a chalky or banded matrix.
Key Diagnostic Tests
- Mohs hardness: 5.5–6.5; it will not scratch quartz and is softer than most chalcedony.
- Streak: white.
- Fracture: conchoidal, with a glassy break.
- No cleavage (amorphous).
- Density: low, ~1.9–2.2 g/cm³ — opal feels notably light for its size.
- Hydrophane test: some Spencer opal is hydrophane and may stick to a damp tongue or change clarity when wet; test cautiously, as water can affect raw opal.
- No acid reaction, no magnetism.
Common Look-Alikes and How to Tell Them Apart
- Lab-created/synthetic opal: shows a regular columnar 'snake-skin' or lizard-skin pattern with a too-perfect, repeating play-of-color; natural Spencer opal is irregular.
- Opalite / opalized glass: man-made glass shows a milky blue-to-orange glow but no true spectral play-of-color.
- Common opal (potch): same mineral but no play-of-color at all.
- Fire opal / Mexican opal: transparent orange-red body color rather than the white-gray Spencer base with flash.
- Plastic imitations: very light, warm to the touch, and the 'color' is printed, not optical.
Where It Is Found
Spencer Opal comes from the Spencer Opal Mines near Spencer, in Clark County, Idaho, where precious opal fills thin seams in rhyolitic volcanic rock. Because the natural layers are so thin, lapidaries assemble triplets to make usable, durable stones.
Frequently asked questions
How can you tell if it's real Spencer Opal?
Real Spencer opal shows shifting spectral play-of-color (often pinfire green and blue), a low density that feels light, and an amorphous conchoidal break. Many genuine pieces are triplets — a thin opal layer between a dark base and a clear cap — which is normal for this deposit.
Why is Spencer Opal usually a triplet?
The natural precious-opal seams at Spencer are extremely thin, so cutters bond the thin opal layer to a dark backing and a clear quartz or glass cap to create a stable, displayable stone.
What does Spencer Opal look like?
A white-to-gray opal that flashes green, blue, orange and red in fine pinfire or broad-flash patterns when tilted under light.
Is Spencer Opal hydrophane?
Some Spencer material is hydrophane and can absorb water, becoming more transparent or sticking slightly to a damp surface. Avoid soaking raw opal, as water can temporarily mute the play-of-color.
Spencer Opal vs synthetic opal — how do I tell them apart?
Synthetic opal shows a regular, repeating columnar or lizard-skin pattern under magnification; natural Spencer opal has irregular, random play-of-color.