The dominant rocks in the area are Devonian sandstones and siltstones, horizontal or slightly dipping. They are cut by dolerite dykes and sills. Clay has formed at the sandstone/dolerite junction, and in veins in the sandstone. The opal is found mostly associated with the clays.

The most important mine in the area appears to be that at Boi Morto, about 3.5 km NNW of Pedro II. The mining is by a series of adits into the hillside, which, above the mining level is a massive sandstone, below which is a friable clay band about 1.5 m thick into which the adits have been driven. Below the clay is dolerite. Jobbins notes that the opal appears to occur as veins in a quartzitic sandstone at the lower surface of the sandstone, suggesting that silicification occurred near the more impermeable clay band, as has been so commonly found in the Australian opal fields.

Of the opal itself, Jobbins (R0186) states:

And further:

The mining appears to be largely by hand from the adits driven more or less horizontally into the opal-bearing layers. The nature of the ground is such that much timbering is necessary; it appears from the photographs that there is plenty of rough timber in the neighbourhood. The ore is brought out in hand barrows, the only 'technology' used being the supply of electricity for lighting from diesel generators.

The general appearance of the rough opal is, in many cases, similar to that mined at Coober Pedy. Some examples of Boi Morto opal are shown.

Numerous other minor localities are described briefly by Jobbins; the general geology and modes of occurrence appear to be similar, sometimes differing in detail.

Regarding the production and quality of Brazilian opal, it has been stated by Sauer (R1600) that: