This odd tumorous-looking thing, on a sandstone boulder in the Oakland Conglomerate in Joaquin Miller Park, is a concretion.
I’ve documented concretions in Oakland before, in rocks of the Great Valley Sequence and in the nameless unit of Eocene mudstone above Shephard Canyon. This concretion is unlike the other two in (I assume) not having a siliceous matrix like the first and not being finely layered like the second. I assume that this is a typical featureless ball of extra-strong mineralization that formed slightly before the rest of the rock lithified. (And on KQED Quest Science Blogs this week, I talk about other concretions in the Bay area and California.)
By the way, I visited the lower end of Joaquin Miller Park the other day, below the Woodminster area where the Miller cottage is, and finally saw my sign about the rocks of the park. I hope that people have gotten some benefit from it.