The Pedernal Hills are an ancient eroded range of mountains consisting of uplifted Precambrian basement rock. The Pedernals themselves are mostly composed of granites with some minor schists and gneisses. The entire Precambrian core of the Pedernals was uplifted during Pennsylvanian times as part of the famous Rocky Mountain
Any search for diamonds in the Pedernal Hills must center on the only known source of the gemstone - kimberlites. There are many definitive associations between ores and rock type in the geologic world. Platinum for example only occurs with basic to ultrabasic rocks such as norite or peridotite. Ni/Cu deposits are likewise only associated with norite. Tin deposits are universally associated with felsic rocks like granite while magmatic corundum seems to be found only in nepheline syenites. Likewise, diamonds are found only in kimberlites.
Kimberlites are vertical, carrot-shaped bodies of ultrabasic rock derived directly from the mantle. They contain many xenoliths of surrounding country rock and consist mostly of greenish-blue peridotite rock which weathers yellow at the surface. Peridotite is comprised of a number of minerals including olivine, phlogopite, pyrope garnet, chrome spinel (picotite), ilmenite, and rarely diamond. Diamond-bearing kimberlites are found only in the oldest and most stable of continental settings. Although the youngest diamond pipes are about 15 million years old, the majority of the world's kimberlite deposits seem to date from an earlier period in earth's history, about 70 to 150 million years ago.
A number of kimberlites, some of which are diamond-bearing, occur in the central part of the United States. The most famous of these is the massive diamond-bearing lamproite pipe at Murfreesboro, Arkansas. This kimberlite cuts through the surrounding Carboniferous and Cretaceous sedimentary rocks which form the country rock in this area. In Kansas, a number of Cretaceous-age kimberlite pipes occur in Riley County, near the town of Stockdale. These kimberlites seem to lack diamonds. In Colorado, the so-called "Stateline" kimberlites crop out near the border of north-central Colorado and Wyoming. These kimberlites are much older than the majority of kimberlites in the world, dating from the Devonian Period or older. They intrude Precambrian granites and metamorphics and are diamond-bearing. In New Mexico, a few kimberlites and lamprophyre dikes occur in the northwest part of the state, but all are devoid of diamonds.
Prospectors should focus their search on the tell-tale weathering product of peridotite, the so-called "yellow ground". (When weathered, the greenish-blue peridotite decomposes quickly to a yellowish mass at the surface.) In addition, placer accumulations of extremely resistant accessory minerals like pyrope and picotite should be noted as they are sure indicators of nearby kimberlites. Interestingly, diamond pipes are generally vertical when emplaced. This would certainly explain the vertical shaft in the cave discovered by the Santa Fe trader.