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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

RI Range1.370–1.470

SG Range1.98–2.25

SG Typical2.15

Hardness (Mohs)5.5–6.5

Crystal SystemAmorphous

Optic CharacterSR (Singly Refractive)

Dispersion0.020

Fluorescence LWVariable

Fluorescence SWVariable

Chelsea FilterInert

PleochroismNone

ColorsColorless, Yellow Orange, Red Pink, Green, Blue Violet, Black, Multi

SpeciesOpal

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.

GemID Pro includes a two-phase natural vs. synthetic testing protocol for Opal.

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Treatments

Smoke/Sugar Treatment (to darken potch)
Oiling / Wax impregnation
Doublet (natural opal + dark backing)
Triplet (thin opal + dark backing + clear cap)

Price Context

Natural — low ($/ct)$50

Natural — high ($/ct)$2,000

NotePer carat; black opal (Lightning Ridge) top value; crystal opal mid-range; white opal lowest

Synthetic — low ($/ct)$10

Synthetic — high ($/ct)$80

Price context is approximate. GemID is not an appraisal tool. Results are indicators, not certified valuations.

Measurement Guides

Identifying an opal? GemID walks through these tests in order — RI, SG, fluorescence, and more.

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Studying for the GIA, FGA, or FEEG practical? Opal is one of the 130 species in GemID’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.

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