Gilson opal is a synthetic precious opal — that is, a laboratory-created material sharing the essential chemical composition and play-of-colour mechanism of natural opal — produced by the Swiss firm Pierre Gilson S.A. and first introduced to the gem trade in the early 1970s. It was among the earliest synthetic opals to achieve genuine commercial scale, and it remains the benchmark against which other laboratory-grown opals are evaluated. Unlike simulants such as plastic or glass imitations, Gilson opal is composed of amorphous hydrated silica (SiO₂·nH₂O), the same fundamental material as natural precious opal, and its spectacular play-of-colour arises from the same physical mechanism: the diffraction of visible light by a three-dimensional lattice of uniformly sized silica spheres. Its importance to gemmology is threefold — it demonstrated that opal's complex microstructure could be replicated artificially; it provided a durable, affordable alternative for jewellery and educational use; and it established the diagnostic criteria by which synthetic opals as a class are now identified.

Formation and Structure

Natural precious opal forms over geological timescales as silica-rich groundwater percolates through voids in host rock and deposits successive layers of silica spheres, which self-organise into a close-packed array. The Gilson process compresses an analogue of this sequence into a laboratory timeframe through a carefully controlled low-temperature precipitation method. Silica spheres of tightly uniform diameter are synthesised in aqueous solution and allowed to sediment and consolidate under moderate pressure, replicating the ordered sphere packing responsible for play-of-colour. The entire process, while far faster than geological deposition, still requires many months to produce material of gem quality — a fact that distinguishes it from rapid-growth simulants and contributes to its relatively faithful optical behaviour.

The resulting material contains a water content of approximately three to five per cent by weight, comparable to the lower end of natural opal's range, and has a refractive index of approximately 1.45 and a specific gravity of around 2.00, both consistent with natural opal. Hardness on the Mohs scale is approximately 5.5 to 6.5, again within the natural opal range. These overlapping physical constants mean that simple refractometer or density measurements alone cannot reliably separate Gilson opal from its natural counterpart.

Optical Properties and Play-of-Colour

The play-of-colour in Gilson opal can be vivid and broad-spectral, encompassing red, orange, green, and blue in the finest grades. Because the silica spheres are manufactured to a highly uniform diameter — far more consistent than those found in most natural opal — the colour patches tend to be bold and geometrically regular rather than the flowing, irregular patterns typical of fine Australian or Ethiopian material. This regularity is itself a diagnostic clue, though it is most apparent under magnification. Gilson produced material in several colour types, including white-base (analogous to white precious opal), black-base (analogous to black opal from Lightning Ridge), and fire-opal-style material, giving the trade a range of appearances to work with.

Gemmological Identification

Distinguishing Gilson opal from natural precious opal requires magnification and, in some cases, transmitted-light examination. Several features are diagnostic:

• Columnar structure: Viewed in transmitted light, Gilson opal frequently reveals a columnar or cellular internal architecture — vertical columns of consolidated silica-sphere arrays separated by boundaries — that is absent in natural opal. This is sometimes described as a "column" or "pillar" structure and is considered the most reliable single indicator.
• Lizard-skin surface pattern: Under reflected light at moderate magnification (typically 10× to 30×), the surface of Gilson opal often displays a fine mosaic or scaly texture resembling lizard skin, caused by the boundaries between consolidated sphere domains. Natural opal does not exhibit this pattern.
• Uniform sphere size: Scanning electron microscopy reveals that the silica spheres in Gilson opal are exceptionally uniform in diameter, a consequence of controlled synthesis. Natural opal spheres, while ordered, show greater size variation.
• Regular colour patch geometry: The play-of-colour patches in Gilson opal tend toward geometric regularity and sharp boundaries under magnification, contrasting with the more organic, flowing colour distribution of natural material.
• Absence of natural inclusions: Natural precious opal frequently contains sand grains, potch (colourless opal), or other host-rock inclusions. Gilson opal is typically inclusion-free in the conventional sense, though its internal columnar boundaries are themselves characteristic.

Experienced gemmologists at the major testing laboratories — including GIA and SSEF — can identify Gilson opal with confidence using standard gemological microscopy. Infrared spectroscopy can provide additional confirmation, as the OH absorption features of Gilson opal differ subtly from those of natural opal in some studies.

Trade Disclosure and Market Context

Gilson opal has always been sold with disclosure as a synthetic material, and Pierre Gilson S.A. marketed it explicitly as such. This transparency was unusual for its era and set a positive precedent for the synthetic-gem trade. In practice, however, Gilson opal has occasionally entered the secondary market without adequate disclosure — particularly in older jewellery pieces where provenance documentation has been lost — making gemmological identification skills relevant to appraisers and dealers handling estate material.

In the contemporary market, Gilson opal occupies a distinct niche. It is popular in costume jewellery and fashion accessories where the visual impact of play-of-colour is desired at a price point inaccessible with natural precious opal. It is also widely used in gemmological education, where its consistent quality and availability make it an ideal reference specimen for teaching opal identification. Fine natural precious opal — particularly top-grade black opal from Lightning Ridge, New South Wales, or high-colour crystal opal from Coober Pedy — commands prices that Gilson opal cannot approach, and the two materials serve largely separate market segments.

Other manufacturers have produced synthetic opals using broadly similar processes — notably Kyocera (whose Inamori synthetic opal became well known in the 1980s and 1990s) — and the gemmological community uses "Gilson opal" both as a proper name for the Pierre Gilson product and, informally, as a near-generic descriptor for synthetic opals of this structural type. Strict usage reserves the name for the Gilson product specifically.

Care and Durability

Like natural opal, Gilson opal is susceptible to dehydration, crazing, and mechanical damage. It should not be exposed to prolonged heat, ultrasonic cleaning, or harsh chemicals. Its water content, while lower than some natural opals, is sufficient to make it vulnerable to thermal shock. Standard opal care guidance — gentle cleaning with a damp cloth, storage away from heat sources, avoidance of prolonged exposure to very dry conditions — applies equally to Gilson material.

Further Reading

• GIA Gems & Gemology — Synthetic Opal (Gilson)
• GIA Gem Encyclopedia: Opal
• International Gem Society — Opal Information