The Alma pyrite mine and its neighbors dug into the hillsides of Leona Heights from the 1890s to the 1930s. The mineshafts have been covered up over the century since, so the fabulous veins of solid pyrite inside the hills are long out of reach. The Oakland park naturalist Paul Covell was leading groups to the old diggings in the 1950s, and collectors reported finding decent pyrite crystals in the mine waste right up to this century, when the last shaft was sealed (but not the last adit).

The rock around the Alma mine is still full of pyrite, which oxidizes to leave earth-tone crusts, the same ocher the Ohlones used to mine here. (Pyrite’s other oxidation product is sulfuric acid, which is what the Americans used to mine it for.) The middle stretch of Redwood Road, from the Lincoln Square shopping center to Campus Drive, could just as well be named Red Rock Road.

I’ve had little luck finding any decent pyrite specimens there. But recently I spent some time at the Alma mine site and found a few bits of ore-grade rock scattered among the barren waste. The site is a little private park on Terrabella Drive, next to a big water tank that sits in the gully where the mine entrance used to be.

The ore-grade bits stand out because they’re silvery, not red, and they’re heavy: the color means they’re fresh and the heft means they have a lot of pyrite, which is twice as dense as the rest of the rock. But in all that red waste, it takes the eyes a little time to see them.

The pyrite grains come in two populations. Some are big enough to show their blocky crystal forms, like in this specimen. They may even give the rock a goldish tinge.

Sometimes these crystals form veins and layers that you can visualize joining into thicker layers and solid masses.

Most of what I found was stones like this, pretty homogeneous and undistinguished at first glance.

But they feel abrasive and have a certain fuzzy sheen. This blowup reveals the cause.

The other population of pyrite grains, then, is dust-sized and disseminated. The big veins were easy to mine and process, but this stuff was trouble.

Pyrite has its name, meaning “firestone,” because it strikes sparks. Humans have used it to make fire since before our own species arose. Pyrite dust oxidizes so eagerly that it’s flammable, and the Oakland mines were plagued by underground fires. Miners would snuff them out by walling off the burning section, then open it up and try again, maybe months later, or give up and dig somewhere else.

The pyrite in Leona Heights may have formed in two different episodes. The fluids that create ore deposits are never exactly the same, and pyrite has room in its crystal structure for lots of different impurities, including arsenic, silver, copper and gold. In fact, the silver and gold that were known to be in the Alma Mine’s ore would be worth recovering at today’s prices. If different sized grains have different chemistries, that would be a good tell, but I have no chemistry lab and can only wonder. Someone else will have to do the analysis. Seems like it would be a good undergraduate project for a geology student.

This entry was posted on 5 January 2026 at 7:40 am and is filed under Quarries and mines. 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.