Oakland Geology

My book Deep Oakland is based on the huge variety of rocks and geologic features we have in this town and near it. In that way Oakland is like California, a state of fantastic geological variety. Indeed, in Oakland you can find more rock types than in any other city in America.

That variety of rocks is intriguing. Does Oakland, like California, have some cool minerals? Do we have any gemstones? The answer to both is “Yes, sort of.”

Oakland has no minerals of precious value except conceivably a sniff of gold.

The classic Gold Rush skipped the East Bay; all anyone noticed along the front of the “Contra Costa Range” was a few warm springs and oil seeps and sulfide deposits. And yet there’s the single report of the short-lived Temescal gold mine on A. D. Pryal’s ranch in the North Oakland hills circa 1864, a story not repeated in any of the local histories.

There were rare reports of gold-bearing quartz elsewhere in the East Bay hills in the 1800s, none of them by reputable geologists. One specimen, though, was supposedly found by a prospector in the Leona Heights area. The landowner he showed it to, Fritz Boehmer, was inspired to dig around his property there, where he found three minerals worth a rockhound’s attention.

Boehmer already knew about the first mineral, one the Ohlones had mined and traded for many centuries: ocher. In fact he had recently finished mining it out to supply red pigment for a paint factory. This stuff.

I consider ocher to be the most valuable mineral in Oakland, given its long history as a resource. It’s also the most collectible, if your collecting tastes run that way.

Digging around, Boehmer encountered a large body of sulfide minerals beneath the ocher. He started a mine and named it for his daughter Alma. The Stauffer Chemical Company built a plant at the Melrose train station to process the ore from the Alma Mine as well as its own Leona Mine nearby. Pyrite, processed to make sulfuric acid, was the pay dirt. It occurred in thick, pure veins. This stuff.

For a while, then, this was the most valuable mineral in Oakland. It’s no longer collectible, though, because all the mine workings have been sealed for many years.

Boehmer’s third mineral was a specimen crusted with sky-blue crystals that caught his eye soon after the Alma Mine opened. He lent it to a friendly mineralogy student at UC Berkeley, who determined the crystals represented a mineral new to science that he named boothite, a story I tell elsewhere. Boothite is rare and delicate; even though Oakland is its type locality, it’s gone and you’ll never find it here. Although keep your eyes peeled at estate sales and swap meets, because there just might be historic specimens from the Alma or Leona Mines in circulation.

For collectible Oakland minerals, that’s pretty much it.

For precious stones in Oakland, other than gold (see above), maybe we have a couple, if you count semiprecious lapidary-type stones.

Some of our serpentine rock might be compact and hard enough to qualify as “California jade,” something like this fist-sized item from a Millsmont back yard.

It would make a handsome pendant or pill box.

Our cherts don’t have the requisite qualities. The younger white chert of the Claremont Shale isn’t hard enough, despite its visual interest. It’s classified as porcelanite, transformed only partway toward the fully microcrystalline silica known as agate.

The older red and multicolored chert of the Piedmont block’s Franciscan melange is hard and lustrous — jasper, really — but not sound enough to stand up to sawing and polishing, although maybe someone out there has tried.

The basalt rocks of Sibley Volcanic Regional Preserve are said to contain nodules of proper agate. (More correctly, the nodules are called amygdules, and they usually consist of other minerals.) Collectors widen their eyes at the mention of Sibley’s blue agates, but I’ve never seen one and the only picture I know of, on Steve Edwards’s wonderful californiageology.net site, is more interesting than impressive. Given that, plus the restrictions on collecting, I suggest respectfully that “Berkeley blue agates” be thought of as legendary.

Are we out of luck, then? Maybe not, if we look beyond conventional stones. I’m not a lapidarist, so maybe my ideas are lame, but how about trying our San Leandro gabbro?

If I had my own rock saw and lapwheel, though, I think my first candidate would be the indistinct blue-green nodules found here and there throughout the Leona volcanics. The matrix is the same hard, sea-changed volcanic ash found everywhere else, with a dash of celadonite accounting for the color.

Anyone want to try these?

This entry was posted on 27 November 2023 at 8:01 am and is filed under Geoheritage. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.