Greenstone
Greenstone cobble cut by a quartz vein.

Greenstone

This cobble consists of a siliceous replacement of once was probably basalt or another volcanic rock that had been metamorphosed into greenstone (green-schist facies). It is cut by a quartz-filled vein. Greenstone is a common replacement of volcanic rock that has been exposed to seawater or brines over a long period of time. This sample is from a gravel bar along Tres Pinos Creek in San Benito County, CA.
https://gotbooks.miracosta.edu/rocks/metamorphic/31.html 2/2/2021