A good description of the Mexican opal fields is to be found in Leechman (R0384). There is some uncertainty as to how long opals have been known from this source, but it appears to have been well known to the earlier civilisations such as the Aztecs. Leechman states that:

It seems likely that the early Spanish explorers must have encountered the stone, but records of it being brought back to Spain are scanty.

The most important area of production is in the region of the town of Queretaro, some 250 km northwest of Mexico City.

The opal occurs in small cavities in fine grained volcanic rocks of rhyolitic or trachytic character. The rock is apparently sufficiently hard for explosives to be necessary to break up the rock which is extracted from small quarries rather than by underground mining. The larger pieces are then broken by hand to extract the fragments containing opal. The opal itself is often shattered by the blasting activities as it tends to adhere strongly to the enclosing rock. Occasionally the opal may be loose in the cavity, but in most cases it needs to be cut clear from the matrix.

Much of the Mexican opal has a reddish or orange body colour through which the diffraction colours, commonly red to green, show. Occasionally milky types of opal are found, as well as some which might be categorised as black opal.

Leechman (R0384) describes an unusual type of opal, known locally as 'contra luz'. He states that:

The reason for this property is obscure, but may be related to a special arrangement of the sphere stacking within the opal, yielding phenomena related to those seen when viewing opal under the microscope between crossed polarisers. More is given in the section Opal and light: Optical properties.

In some cases nothing more than the orange to red body colour is found, with no diffraction colours evident. Such material, because of its inherent colour, retains some value and the stones may be faceted to give them more character.

Most of the cutting into cabochons appears to be done by local cutters using simple equipment. The matrix rock is ground away, the stone rough ground, then finished in the usual way by fine grinding followed by polishing on a buffing wheel. The lapidarists also cut other types of stones such as malachite, agate, and turquoise. Leechman also tells us that: