Sandstone samples from the beach just South of Morro Bay, California.

The rocks form Montano del Oro are mostly "porcelanite" because it looks like unglazed porcelain. The Pismo Formation outcrops in that area. It is related to the Monterey Formation which was deposited along almost the entire northern Pacific Rim during the late Miocene (17-5 million years ago). The sediments that were deposited were mostly composed of diatoms. When buried the diatoms (made of silica) dissolve and reprecipitate into stable forms of silica including quartz. If it was a very pure silica, the new rock would be chert - hard and glassy. If it is less pure (with clay mixed in) then it will form porcelanite, like your samples.