Every semester I get questions from students at College of Marin about where they can find carnelians. They may have sifted through the chert pebbles at Rodeo Beach, in the Marin Headlands, in hope of spotting one of the elusive orange gems, but in most cases they’ve come up empty-handed. But they’ve heard there are carnelians out there, and they ask if I can tell them how to find them. I’m afraid the answer is yes–and no.
Carnelian is the name given to the reddish variety of chalcedony, which is itself a variety of the mineral quartz, which is formed of SiO2. It makes sense that we should find it among the vast cherts of the Marin Headlands, for these cherts are primarily composed of SiO2, and it is not a stretch of the imagination to see how liberated SiO2 becomes recrystallized into new forms.
Chalcedony is cryptocrystalline quartz, meaning that the minerals exist but are vanishingly small. In thin section chalcedony often presents as parallel fibers, but this structure is so small that in hand sample it appears dull and waxy. The transparency of quartz is present, however, and if small amounts of iron contaminants are incorporated as the crystal grows, it will take on a pleasant orangish-reddish hue. This is carnelian.
Carnelians have been used in jewelry for millennia, and many examples were found in the treasures of King Tut. One particular use for carnelians was in signet rings–used to press a symbol into hot wax, in an archaic kind of signature–because wax does not stick to carnelian.
One of the mysteries about the carnelians found at Rodeo Beach is their source. Rodeo Beach meets the Pacific at the mouth of the long valley that forms the merger of Gerbode and Rodeo valleys. Perhaps somewhere up there is an exposure of carnelian-rich rock?
Geologists and rock hounds have thoroughly tramped these hills in vain looking for this source rock. If we find carnelians at the beach, surely they must come from somewhere higher. It turns out that someone knows exactly where the carnelians come from.
My Top Secret Source™ revealed to me that years ago, following an exceptionally violent storm surge, the sand at Rodeo Beach was so throughly removed that the passage between Rodeo Lagoon and the Pacific Ocean opened up. This is near the edge of the parking lot, where one now walks down the steep embankment toward the sand. And there, deep under what is normally covered by the chert pebbles of Rodeo Beach, was an outcrop rich in carnelians. Yet just as swiftly as it appeared, the sand once covered the carnelian source rock. From the description of my Top Secret Source™, I think the outcrop is in the center of this picture:
It must be that every few decades, a storm is violent enough to expose and erode the carnelian source rock, liberating a burst of carnelians to mix with the countless chert pebbles at Rodeo Beach. Over the millennia, this led to a surplus of carnelians at Rodeo–a surplus which, when humans descended upon the beach and began actively hunting for them, disappeared so thoroughly that now only the tiniest carnelian fragments, some no bigger than a grain of dust, can be easily found.
So when my students ask how they can find big carnelians, the answer is you usually can’t. But that condition might change immediately after a vigorous storm, which tends to churn up much sand and reveal hidden gems. Or if the storm is exceptional, the source rock itself might make a brief appearance. For most of the time, though, you’ll just have to appreciate what Rodeo Beach has in abundance: beautiful chert fragments honed by nature’s own rock polisher.