I found this in the bed of the Mad River in northern California. My guess would be that its some sort of quartz deposits on a sandstone, but that is just my novice guess. Those little bubble like forms have the white inside and that is why I thought they might be quartz. It is about 8 1/4 inches. I liked the shape so brought it home to add to my collection but would like to know a little bit more if anyone can help.