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Fossil burrow, with black rock inclusions that might include part of a fossilized shrimp claw.
Found in Tres Pinos Creek bed. I really do not know what this cylindrical rock is, but there are many similar Ghost Shrimp burrow structures in the younger Purisima Formation fossils at nearby New Brighton State Park: http://www.idigfossils.org/wp-content/uploads/2018/06/Perry-1993-CapitolaFossils.pdf. The rock is hard and river tumbled. It is 2.5 inches long, 1.75 inches wide and weighs 5.75 ounces. There is a thin, reddish brown outer cylindrical sandstone layer filled with yellower sandstone that has black inclusions. It is not attracted to a magnet. Most of these images were taken with my iPhone. The last two images were taken with my Sony RX10IV camera. They are of better quality and I washed the rock of loose dust. A previously unseen concentric ring of dark material was uncovered at one end by the cleaning.

Solitary form of P. confoedarata. You can see the coiled up stolon comprising the more commonly noticed clonal chain-morph. This is the type that creates the clones, the clones then reproduce sexually. Huge, about a foot long.