Oiling

Oiling is the filling of surface-reaching fissures with oil or resin so that they show less. In emerald it is standard practice, and has been since Colombian rough first reached Europe; the trade accepts it without comment provided it is disclosed. Cedarwood oil is the traditional filler. Resins such as Opticon and Permasafe came later and hold better, which is why the type of filler matters as well as the amount. An oiled emerald is not treated in the way a diffused sapphire is treated. It is emerald in its normal commercial state, and "no oil" is the rare positive claim, not the baseline.

On a lab report

Laboratories grade the amount of filler, and the grade moves price. SSEF and Gübelin write "insignificant", "minor", "moderate" or "significant" against a phrase such as clarity enhancement or amount of oil. GIA writes "indications of clarity enhancement" and qualifies it as minor, moderate or significant. Where the filler is resin rather than oil the better houses say so, since resin is harder to remove and usually points to fissures that needed more help. "No indications of clarity enhancement" is the line that commands the largest premium, and it is rare enough that a dealer will lead with it.

Oil is not permanent. It can dry, sweat under a jeweller's torch or leach out in an ultrasonic bath, and re-oiling between owners is routine and unremarked. That is why the grade belongs on a current report and not on one issued a decade ago, and why an emerald that has passed through several hands deserves a fresh look before money moves. Ask for the filler grade in writing, ask what was used, and ask when the report was issued. Then hold the stone under a lamp and look along the fissures for the faint flash of colour that resin sometimes gives.