Opticon filling is the trade name for the use of Opticon, a proprietary epoxy resin produced by Hughes Associates, to fill surface-reaching fissures in emeralds. The treatment improves apparent clarity by reducing the visibility of fractures, since the resin's refractive index (approximately 1.55 to 1.56) is close to that of emerald (1.577 to 1.583). Opticon is applied under vacuum or pressure to draw the liquid resin into the fissures, then cured into a polymer network by mild heat. The treatment is partially reversible: prolonged heat, ultrasonic cleaning, or aggressive solvents can degrade or remove the resin, and exposure to ultraviolet light may yellow the polymer over time. Laboratories classify Opticon filling as 'minor' to 'moderate' enhancement depending on the quantity present.

The application process

The standard application of Opticon involves several controlled steps. The emerald is first cleaned thoroughly to remove any oil, dirt, or organic contaminants from the existing fissures and the surface. Common cleaning methods include solvent rinses, ultrasonic cleaning in mild detergent, and warm-water soaking; the goal is to leave the fissure network completely free of any pre-existing oil or other filler that would interfere with the Opticon penetration.

The cleaned stone is then placed in a vacuum chamber with the prepared Opticon mixture (resin and hardener combined in the manufacturer-specified ratio). The chamber is evacuated, drawing air out of the fissures within the stone. Releasing the vacuum pressure allows atmospheric pressure to drive the Opticon liquid into the now-empty fissures, with the low viscosity of the resin allowing it to penetrate deep into the stone's internal fissure network.

The stone is then heated mildly — typically to 60 to 80 degrees Celsius for several hours — to cure the resin in place. The heat catalyses the cross-linking reaction between the resin and hardener, producing a stable polymer network that fills the fissure space and bonds chemically rather than remaining as a free-flowing liquid. After curing, the stone is cleaned again to remove any excess resin from the external surfaces.

Comparison with traditional oil treatments

The traditional treatment for emerald fractures is filling with cedarwood oil — a natural, transparent, oil-based filler with a refractive index reasonably close to emerald's. Cedarwood oil has been used for centuries in the emerald trade and is still considered the most commercially acceptable treatment in much of the trade. Its principal advantages are its non-toxicity, biodegradability, easy removal for re-treatment, and traditional acceptance.

The principal disadvantage of cedarwood oil is durability: oil evaporates and migrates out of fissures over time, requiring periodic re-oiling — typically every few years — to maintain the apparent clarity. Stones in active use may need re-oiling more frequently. The maintenance burden falls on the owner or the retailer providing service.

Opticon is more durable than cedarwood oil. The cured polymer remains in place for much longer than oil, with normal wear and cleaning conditions producing little degradation over years or decades. The principal disadvantages are that the polymer is harder to remove (complicating any subsequent re-cutting or re-treatment), that it may yellow under sustained UV exposure, and that some markets consider polymer fillers a more significant treatment than natural oil.

Other polymer fillers

Opticon is one of several polymer fillers used in the emerald trade. Other branded products include Permasafe, Lab/Opticon variants, and various proprietary formulations from emerald-treatment specialists. Each has slightly different chemistry, refractive-index match, durability characteristics, and reversibility properties. Laboratory infrared spectroscopy can typically identify the specific filler type from its characteristic absorption features.

The choice between Opticon and other polymer fillers depends on the specific application and the treater's preferences. For high-end fine-jewellery emeralds where reversibility and aesthetic match are critical, more sophisticated polymer formulations or natural cedarwood oil may be preferred. For commercial-grade material where durability is the priority, Opticon and similar products are widely used.

Reversibility

Opticon filling is partially reversible. Heat treatment at moderate temperatures can soften the polymer enough to allow it to flow out of the fissures, although the process requires careful control to avoid damaging the stone. Solvent treatment with strong organic solvents (acetone, methylene chloride) can dissolve the cured resin, although prolonged exposure may also affect any residual oil from prior treatments and may not be fully effective on deeply embedded resin.

For practical purposes, Opticon-filled emeralds can be 'reset' by careful treatment in a specialist laboratory, allowing re-treatment with fresh resin or with a different filler. The reset process is non-trivial and is generally undertaken only when the existing treatment has degraded substantially or when the stone needs to be re-cut.

In the trade

For working dealers and jewellers, Opticon filling is a recognised, legitimate, and widely used emerald treatment. Disclosure is mandatory: any sale or display of an Opticon-filled emerald must include accurate disclosure of the treatment status, ideally supported by laboratory documentation that identifies both the filler type and the quantity of filling. Customers buying commercial-grade emeralds should expect some level of filling, since virtually all emeralds entering the international trade have been treated to some degree, and should evaluate the laboratory description (minor, moderate, significant) when assessing quality and pricing.

See also Opticon emerald, cedarwood oil, emerald enhancement, and emerald for related entries.

Further reading

• GIA Gems & Gemology — emerald enhancement and laboratory practice
• AGTA — coloured stone treatment terminology
• International Gem Society — emerald treatment reference