Opal

Related: Opal Varieties

Black OpalFire Opal

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.

Treatments

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

Price Context

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

About Opal

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).