Moderate oil, abbreviated O2 in laboratory shorthand, is the middle of three principal levels of emerald clarity enhancement recognised by the Laboratory Manual Harmonisation Committee (LMHC). The O-scale runs from O1 (minor or insignificant), through O2 (moderate), to O3 (significant or prominent), with the additional designation of none for unenhanced stones. Almost all of the world's emerald supply ships with some level of fissure filling, and O2 is the most common laboratory-issued grade in commercial production.

The LMHC scale and what O2 means

The LMHC was formed in 2001 by representatives of leading coloured-stone laboratories, including Gubelin, SSEF, GIA, AGL, GIT, and CISGEM, with the aim of harmonising the language used on coloured-stone reports. The committee published agreed wording for clarity enhancement in emerald, settling on the simple O-scale as a way to communicate the extent of fissure filling without committing to a specific filler chemistry on the front of the report.

O2, moderate, denotes a level at which the filler is making a clear and useful contribution to the apparent clarity of the stone. Under magnification, an O2 emerald will show fissures with visible filler — sometimes with flow lines, gas bubbles, or a characteristic flash effect at the boundaries — and the gemmologist judges that those fissures would be clearly less attractive without the filling. The grade is descriptive rather than prescriptive: it tells the buyer what the laboratory observed, not whether the treatment was performed well or badly.

Relationship to GIA's F-scale

GIA uses an internal F-scale (F1, F2, F3) that maps to the LMHC O-scale almost identically: F1 corresponds to O1, F2 to O2, F3 to O3. The terminology differs only in the letter prefix, with GIA using F for filling and LMHC using O for oil. Trade clients can read the two scales as equivalent for practical purposes, although careful buyers will note that the laboratories sometimes disagree by one step on borderline stones.

Acceptance and disclosure

Moderate oiling is widely accepted in the trade as a routine treatment, provided it is disclosed. The 1973 conference of CIBJO and subsequent codes of practice from AGTA, GIA, and the Jewelers of America have established disclosure of clarity enhancement in emerald as a baseline professional obligation. A merchant selling an emerald without disclosing oiling — at any level — risks consumer-protection actions and reputational damage.

Buyers comparing two emeralds of similar size and apparent colour should ask for the laboratory grade. A stone described as O1 commands a higher price per carat than a comparable O2; an O3 trades at a discount. The gap between O2 and a confirmed unenhanced stone of similar quality can be substantial, particularly in the larger sizes where unfractured rough is rare.

Further reading

• LMHC — emerald clarity enhancement information sheet
• Gubelin Gem Lab — coloured stone reports
• AGTA — gem treatment disclosure
• GIA — Emerald clarity enhancement reference