Mining in California means gold, right? Well, not necessarily. The earliest commercial (in the modern sense) mine to operate within the state of California appears to have been the New Almaden, and it produced cinnabar – mercury ore. It was mined as a pigment by Antonio Sunol and Luis Chabolla as early as 1824, but mercury reduction did not begin until 1846. That was still two years before James W. Marshall discovered gold at Sutter’s Mill.
Both gold and silver mining in the nineteenth century depended upon mercury to recover the precious metal from the ore, and more than half the U.S. production (57%) of this vital metal up through 1893 came from the New Almaden. There is a great contemporary description of this mine in Scenes of Wonder and Curiosity in California, written in 1862 by James M. Hutchings. At this time Hutchings wrote the mine had been shut down by a court order, but it soon reopened. More information is available in Will Meyerriecks book Drills and Mills, Precious Metal Mining and Milling Methods of the Frontier West.
As it happens, however, Sunol and Chabolla were not the first people to mine cinnabar at New Almaden; the deposit was being tapped long before the Spanish even arrived in what would become California. And even at that, it may not have been the oldest mine.