Opal
Last updated: April 2026
Precious opal exhibits play-of-color from microscopic silica sphere diffraction. Common opal (potch) lacks play-of-color and appears milky, opaque, or translucent in a single body color — common opal is often misidentified as glass, chalcedony, or moonstone. If no play-of-color is visible, consider common opal or another gem species entirely. Synthetic producers include Gilson (France) and Kyocera (Japan, 'Crescent Shire'). Ethiopian Welo opals are hydrophane and may absorb oil, altering play-of-color; check by submersion in water (reversible enhancement).
Opal has a specific gravity of 2.15 and a refractive index of 1.370–1.470, with a Mohs hardness of 5.5–6.5. It is singly refractive.
Physical & Optical Properties
Measured with a polariscope, a refractometer, a hydrostatic balance, a Chelsea filter, a UV torch, a loupe, a darkfield loupe and a spectroscope — step-by-step for each.
Through the refractometer
Drawn from the refractive index above, on a 1.40–1.81 scale. How to take this reading.
Related: Opal Varieties
Key Differentiators
- Play-of-color
- Low specific gravity
- Amorphous structure
- Common opal (potch): no play-of-color — milky or translucent single body color; may be misidentified as glass or chalcedony
Natural vs. Synthetic
Synthetic opal is commercially available (Gilson synthetic (columnar growth)). Distinguishing natural from synthetic typically requires microscopic examination of internal features.
- Play-of-color pattern (magnification): Random, irregular patch sizes and shapes. Boundaries between color areas are soft and diffuse. Uneven distribution with possible dead spots. Organic and varied. Synthetic: Gilson: 'chicken wire' or 'lizard skin' pattern — very regular, uniformly sized hexagonal/polygonal color patches with sharp straight boundaries. Too perfect and repetitive.
- Side examination in water (magnification): Amorphous, irregular internal structure. No organized columnar pattern. Synthetic: Gilson: distinct parallel columnar structure visible when viewed perpendicular to surface — like tiny parallel bundles of light pipes. Reliable diagnostic for Gilson synthetic.
- Assembled opal — join line: Solid opal: no horizontal join line at girdle. Uniform luster and texture on all surfaces. Synthetic: Doublet: single horizontal cement line at girdle (view in water from side). Luster difference top vs. bottom. Triplet: two join lines (opal sandwiched between dome and backing). High rounded dome profile. Air bubbles at glue layers. Plastic dome shows softer luster and scratches easily.
- UV Fluorescence: Variable — inert (Australian), or weak green/yellow/blue. Some rare Virginia specimens show chalky white (but rare). Synthetic: Gilson: strong, chalky white to bluish-white under both LW and SW UV. Very consistent. Supports but does not alone confirm synthetic origin.
Loupewise Pro includes a two-phase natural vs. synthetic testing protocol for Opal.
Treatments
- Smoke/Sugar Treatment (to darken potch)
- Oiling / Wax impregnation
- Doublet (natural opal + dark backing)
- Triplet (thin opal + dark backing + clear cap)
Measurement Guides
Identifying an opal? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.
Try Loupewise Free →Studying for the GIA, FGA, or FEEG practical? Opal is one of the 132 species in Loupewise’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.
Practice exams →