Treatments: Mexican opal should be “seasoned” by leaving it open to the air for up to two years before cutting to assure stability. This assures the cutter that the stone will not suffer from crazing or turning milky after it is finished. Some unscrupulous sellers of Mexican opal soak their opal in water to temporarily re-hydrate the stone. Opals may also be oiled and opticoned to reduce the visibility of flaws. These are not considered acceptable treatments.
Polishing: Cerium on most common polishing laps.
Comments: Care should be taken when dopping to prevent damage to the stone by thermal shock. Stones that display opalescent fire are usually better to cut as cabochons.