Rock IdentifierOpal
Key takeaways
- Opal is a hydrated silica mineraloid — it contains water and has no crystal structure.
- 'Play-of-color' (flashing spectral hues) is different from ordinary body color.
- Doublets, triplets, and synthetic opal are common; opal can crack if it dries out.
A gem made of tiny spheres
Opal is hydrated silica (SiO₂·nH₂O) — silica plus water, typically 3–10%. It's a mineraloid, lacking the orderly crystal lattice of true minerals. Precious opal's magic comes from microscopic silica spheres stacked in a regular grid that diffracts light into flashing spectral colors, called play-of-color. 'Common opal' (potch) lacks this and shows only a solid body color.
How to recognize it
The tell is play-of-color: tilt the stone and watch flashes of red, green, and blue dance across it, shifting with angle — quite different from a fixed body color or simple sheen. Opal is fairly soft (5.5–6.5) and often has a milky, waxy, or glassy look. Black, white, and translucent 'crystal' opal describe the background, not the flash.
Assembled & synthetic stones
Because fine opal is thin and fragile, it's often assembled: a doublet glues a slice of opal to a dark backing; a triplet adds a clear protective cap on top. Viewed edge-on, these show flat glued layers. Lab-grown (synthetic) opal also exists, sometimes betrayed by an unnaturally regular, columnar 'snakeskin' color pattern. None are 'fake' if disclosed, but they change value sharply.
Care
Opal's water content makes it sensitive: sudden heat, very dry air, or harsh cleaning can cause it to craze (develop fine internal cracks). Keep it away from ultrasonic cleaners and extreme conditions, and store solid opal so it doesn't dry out. Handle it as the delicate stone it is.
