Rainbow Opal Identification Guide

What Rainbow Opal Looks Like

"Rainbow opal" is a marketing name for precious opal that shows a strong, multicolored play-of-color — flashes of red, green, blue, and orange that shift as the stone or light moves. Opal is hydrated amorphous silica (SiO₂·nH₂O); the color comes from diffraction off an ordered array of microscopic silica spheres.

• Color: body color may be white, gray, black, or transparent; the play-of-color is the key feature
• Luster: subvitreous to waxy or resinous
• Transparency: transparent to opaque
• Habit: amorphous (non-crystalline); seen as nodules, seams, and polished cabs

Step-by-Step Field ID Checklist

1. Rock the stone under light. Genuine play-of-color appears as distinct patches and flashes of pure spectral color that move and switch on and off — not a single sheen.
2. View from multiple angles. Real opal's color pattern changes shape and disappears at some angles.
3. Look for natural patch structure (broad flash, pinfire, harlequin) rather than the regular speckle of plastic imitations.
4. Check for a flat join line that would betray a doublet or triplet.
5. Test hardness gently on an inconspicuous spot.

Key Diagnostic Tests

• Mohs hardness: 5.5–6.5 — softer than quartz; handle carefully.
• Fracture: conchoidal, no cleavage.
• Streak: white.
• Specific gravity: low, ~1.9–2.3 — opal feels lighter than glass or quartz of the same size.
• Acid: no reaction.
• Hydrophane test: some natural opal sticks slightly to a wet tongue or temporarily clears when soaked (use cautiously).

Common Look-Alikes and How to Tell Them Apart

• Opalite / "rainbow" glass: shows a uniform milky blue glow and orange transmitted color, no discrete moving color patches, and conchoidal glass fracture. Opalite is the most common fake.
• Synthetic opal (Gilson/lab): play-of-color is too regular, often with a "lizard-skin" or columnar snakeskin pattern visible under magnification, and a perfectly even color distribution.
• Doublets/triplets: look at the side profile for a straight glue line and a black backing or domed glass cap.
• Labradorite: shows a single metallic flash (blue/gold) from a feldspar with cleavage and hardness 6–6.5, not discrete spectral patches.

Where It Is Found

Precious "rainbow" opal comes chiefly from Australia (Lightning Ridge, Coober Pedy), Ethiopia (Welo), Mexico, and Brazil, deposited from silica-rich groundwater in sedimentary and volcanic host rocks.

Collector's Notes and Common Mistakes

The defining test for any "rainbow opal" is play-of-color versus sheen: true opal throws separate, mobile patches of pure spectral color that wink on and off, while imitations show one continuous milky glow. The most frequent fraud is opalite (glass), followed by assembled doublets and triplets — always inspect the stone in profile for a flat glue seam and a dark backing. Lab-grown (Gilson) opal is harder to spot but reveals an unnaturally ordered "snakeskin" or columnar color pattern under magnification. Remember that some natural opal is hydrophane, meaning it absorbs water and can temporarily lose its fire or even crack — never soak unknown opal indiscriminately, and keep Ethiopian material away from oils and dyes. Because opal is soft (5.5–6.5) and can craze (develop fine surface cracks) if it dries out, store it away from heat and direct sun. When grading, value broad, bright, multi-directional play-of-color over a single narrow flash.