Flash Opal Identification Guide

What Flash Opal Looks Like

"Flash opal" is a trade term for precious opal whose play-of-color appears as one or two broad sheets of color that wash across the stone as you tilt it, rather than the discrete dots of pinfire or the mosaic of harlequin. The base is hydrated amorphous silica (SiO2·nH2O), so the stone is non-crystalline with a smooth, waxy-to-vitreous luster on polished faces and a greasy look on rough.

• Color: body tone ranges from white/milky (light opal) through grey to near-black; the flash itself is most often blue or green, less commonly orange/red.
• Transparency: translucent to semi-opaque; crystal-opal varieties can be quite transparent.
• Form: massive, never in crystals; rough comes as nodules, seams, or fillings in ironstone or sandstone host.

Step-by-Step Field-ID Checklist

1. Rock it under a single light source. A true flash sweeps as a continuous broad plane of color that switches on and off at a particular angle. Ordinary surface sheen does not change hue.
2. Check the body tone against a neutral background to classify it (light, grey, black).
3. Inspect the edge/girdle for a glued seam — a flat join line means a doublet or triplet, not a solid flash opal.
4. Look for the silica "feel": waxy luster, conchoidal chips, no cleavage planes.
5. Estimate hardness lightly — opal is soft and scratches easily.

Key Diagnostic Tests

• Mohs hardness: 5.5–6.5. A steel knife will scratch it; it will not scratch quartz (7).
• Streak: white.
• Fracture: conchoidal, no cleavage.
• Density: low, ~1.98–2.25 g/cm3 — it feels noticeably lighter than glass or quartz of the same size.
• No acid reaction, non-magnetic.
• UV: many opals show greenish-white fluorescence.

Common Look-Alikes and How to Tell Them Apart

• Opal doublet/triplet: the play-of-color is real opal, but it is a thin slice glued to a dark backing (and often a clear quartz/glass cap). Tilt to find an abrupt flat seam at the girdle and a perfectly even color layer of constant thickness.
• Lab-created (synthetic) opal: shows a tell-tale "columnar" or snakeskin-lizard pattern and an unnaturally regular color-pattern repeat; flashes look too uniform.
• Opalite / opalized glass: displays a milky blue-white adularescent glow but NO true spectral play-of-color, and it is harder/denser (it is man-made glass).
• Labradorite: has metallic schiller (labradorescence) but is a feldspar with cleavage and hardness 6–6.5, and the color flash is confined to flat crystal planes.
• Moonstone: shows a floating bluish adularescence, not rainbow flashes, and is harder.

The single most reliable separator from cheaper imitations is the combination of broad rainbow flash + low density + soft, waxy silica feel + no glued seam.

Where Flash Opal Is Found

Most gem flash opal comes from the Australian fields — Coober Pedy and Andamooka (light/crystal), Lightning Ridge (black), and Queensland boulder fields. Ethiopian Welo deposits produce bright flash material that is often hydrophane (it can absorb water and temporarily lose flash). Brazil (Pedro II) and Mexico also yield flashy precious opal. In the field it is recovered from weathered sandstone, ironstone, and clay seams.