Oil treatment is the practice of filling surface-reaching fissures in a gemstone — overwhelmingly emerald, occasionally other beryls and certain other species — with a colourless oil, wax, or polymer resin to reduce the visibility of those fissures and improve the apparent clarity of the stone. The treatment works because the refractive index of the filler is closer to that of the host gem than air is, so the optical contrast at the fissure walls is reduced and the eye no longer registers the fissure as sharply. The treatment has been part of the emerald trade for centuries; the modern question is not whether to oil but at what level, with what filler, and with what disclosure.

Materials in use

Traditional cedarwood oil — pressed from the wood of Cedrus species — has been the workhorse filler for emerald for at least the late nineteenth century. It penetrates fine fissures readily, has a refractive index near 1.51 (close to emerald's 1.57-1.58), and is colourless to pale yellow. Its drawbacks are that it can leak under heat or vacuum, evaporate slowly over years, and yellow with age, requiring periodic re-oiling.

Lighter mineral oils and paraffin are also used. From the 1980s onward, polymer resins — most prominently Opticon, a colourless epoxy resin — have offered a more durable alternative, but their use is reported on laboratory documents at a higher tier of disclosure than colourless oil because the treatment is harder to reverse and harder to detect at low concentrations.

Coloured oils and dyes are a separate, more aggressive treatment that imparts colour as well as filling fissures, and is treated by laboratories as a significant rather than minor enhancement. They are generally regarded as outside the bounds of acceptable trade practice when undisclosed.

Disclosure standards

Laboratories following the Laboratory Manual Harmonisation Committee (LMHC) protocols report the level of clarity enhancement on a four-step scale: none, minor, moderate, and significant. The standard is shared by Gübelin, SSEF, AGL, AGTA, GIT, GIA Thailand, and CISGEM. Each laboratory uses microscopic observation, infrared spectroscopy, and ultraviolet fluorescence to make the determination. The wording on the report is consistent across the participating labs and is now the common reference point for the international emerald trade.

CIBJO Blue Books and the FTC require disclosure of treatment at point of sale. For emerald specifically, the trade convention is that the seller will state the LMHC tier where a laboratory report exists, or describe the treatment level in plain language otherwise. Failure to disclose is grounds for return, refund, and reputation loss.

Effect on value

Untreated emerald — particularly fine Colombian material with documented provenance — commands very substantial premiums over treated equivalents, often 50 to 200 per cent above stones of comparable colour and clarity but with moderate or significant treatment. Minor treatment is regarded as the normal commercial state and is priced accordingly. Moderate treatment carries a more visible discount, and significant treatment a deeper one still. Resin-filled stones generally trade below oiled equivalents at the same descriptive tier because of the more permanent nature of the treatment and trade preference for reversible enhancement.

Care

Oiled emerald should not be cleaned in ultrasonic or steam cleaners, both of which can drive oil out of the fissures. Mild soap and warm water is the standard recommendation. Oiled stones in long-term wear typically need re-oiling every several years to maintain appearance.

Further reading

• GIA Gems & Gemology — emerald clarity enhancement
• Gübelin Gem Lab — emerald reporting standards
• AGTA — coloured stone treatment terminology
• CIBJO Blue Books — disclosure standards