Synthetic Opal

Synthetic opal can be produced by the Stöber process, which uses organic solvents not present where natural opal forms; this differs from natural opal’s path of silica-gel solidification.

Scope: The source is a GIA review and states only the synthesis route versus natural formation; no process parameters are given.

Play-of-colour in synthetic opal has the same origin as in the natural stone: monodisperse nano-scale silica spheres packed into an ordered three-dimensional diffraction grating (a photonic crystal) diffract white light and split it into spectral colours under the Bragg condition; the colours seen depend on sphere diameter and spacing (on the order of 250 nm). Ordered spheres of this kind can be built artificially by sol-gel routes such as the Stöber process, yielding material with the same effect.

Scope: Explains the physical origin of the play-of-colour; the details of different synthetic processes, and the naming question of 'synthetic versus imitation', are separate matters, and the mechanism does not establish naturalness or provenance.

Synthetic opal also commonly shows a secondary structural effect: after the silica spheres settle, drying of the silica gel causes 'polygonization' into columnar domains, and repeated generations of it produce the 'lizard-skin' optical pattern typical of synthetic opal, an effect rarely seen in natural material.

Scope: Describes a common structural feature of synthetic material; its presence varies with the process and is not used to identify a single stone.

Synthetic opal, an imitation and natural opal cannot be told apart by play-of-colour alone. In one investigation the split between a synthetic product and a simulant turned on whether the material holds water and matches natural opal's composition, and the authors also set out gemological properties by which such a man-made play-of-colour material can be separated from natural opal. Applying any of this needs laboratory measurement, and the conclusion should come from a qualified body rather than from visual inspection.

Scope: Applies to distinguishing synthetic opal, imitations and natural opal; the determination requires laboratory measurement and should be issued by a qualified body.

Synthetic opal is a laboratory-grown material whose chemistry, physics, and structure are ostensibly those of natural opal; its status as synthetic depends on whether its composition truly matches natural opal—Kyocera’s play-of-color products, containing no water, were classed as opal simulants, not synthetic opal.

Scope: Based on GIA’s assessment of Kyocera products; classification requires verifying composition (water) and structure in a laboratory.

Synthetic opal, too, shows polygonization; when this process happens in several generations it produces the “lizard-skin” optical effect that is typical of synthetic opal yet rarely observed in natural opal.

Scope: Describes a synthetic-opal structural/optical feature used for identification, not a natural formation process.

In identification, the presence of matrix between play-of-color columns may be a first step in recognizing a natural opal; when matrix is absent, other features such as inclusions are needed to separate it from synthetic opal.

Scope: This is an identification clue rather than a definitive test; the authors stress combining it with inclusion evidence.