Mexican fire opal is a transparent to translucent variety of common opal whose body colour ranges from saffron yellow through orange to a strong reddish orange. It is the type-locality material for what the international trade calls fire opal, a name applied to any vivid warm-coloured transparent opal regardless of origin, but Mexico has been the dominant commercial source since the nineteenth century. The principal mines lie in the states of Queretaro, Jalisco, Guerrero and San Luis Potosi, in rhyolitic volcanic host rock of Tertiary age.

Chemically, the stone is hydrated silica with two to ten per cent water by weight. Refractive index sits between 1.42 and 1.45, specific gravity is around 1.97 to 2.10, and hardness is five and a half to six and a half on the Mohs scale. Most Mexican fire opal lacks the diffraction phenomenon known as play of colour, although a minority of stones, particularly from Queretaro, do show flashes of green and red against the orange body. Such material is sold as precious fire opal and commands a substantial premium.

The two principal stability concerns are dehydration and crazing. Mexican opal is more prone to losing water than the sedimentary Australian opal, and stones removed from humid storage and exposed to dry air, direct sunlight or warm display lighting can develop a fine network of surface fractures within months or years. Reputable dealers store rough underwater or in oil and acclimatise stones slowly. Buyers should be advised against ultrasonic and steam cleaning, and against prolonged dry display.

Cutting is almost always faceted when material allows, which is unusual among opals and is a direct consequence of the high transparency. Cabochon cuts are reserved for translucent or play-of-colour material. The Queretaro mines produce the most saturated red-orange material, traditionally called cherry opal in the trade. Imitations made from glass, plastic and from infused common opal are documented, and laboratory disclosure on treatment is increasingly standard.

The cultural reach of the stone in Mexico is significant. It was worked by pre-Columbian artisans and remains a national gemstone in commercial and collector contexts, with denomination of origin protection extended to Magdalena, Jalisco material in the Mexican system.