L is the enhancement code used by the American Gem Trade Association (AGTA) to designate lasering, the use of a focused laser beam to drill or modify a gemstone. The AGTA system, published in the AGTA Gemstone Information Manual, assigns single-letter or short-letter codes to recognised gemstone treatments to support consistent disclosure across the trade. L sits alongside other AGTA codes including B (bleaching), C (coating), D (dyeing), F (filling), H (heating), I (impregnation), O (oiling), R (irradiation) and U (no enhancement).

Laser drilling is most commonly applied to diamonds, where a laser is used to cut a microscopic channel from the surface to a dark inclusion (a black diamond crystal or graphite inclusion), allowing acid bleaching to remove or lighten the inclusion and improve the apparent clarity of the stone. The drill hole itself is a permanent feature, visible under magnification as a fine threadlike channel, and is disclosed as treatment under AGTA, GIA and CIBJO standards.

A variant technique, KM (kiduah meyuhad in Hebrew, special drilling, sometimes anglicised as internal laser drilling) creates a fracture-feathered channel rather than a clean drill hole and is also captured under the L code. Laser inscription on the girdle (e.g. GIA report numbers) is technically also lasering but is conventionally treated as a non-disclosure-required identification mark rather than a clarity treatment, and appears separately on grading reports.