Quartizite in metamorphic rock. This was gravel and clay that got heated and fused into a massive rock. The rocks are hard and brittle and wold rather shatter than come loose. This quartz pebble is much harder than the the matrix that it is in, and it is quite attached, a kick could knock it loose, but it took a few thousand years to get this way, everything around it erroded away. This is near the Greenville geological fault North of Livermore California.