The Clear Lake area about an hour and a half North of SF has a ton of obsidian laying around altho you already have some. The gold country area is very interesting especially if you get off the highways and explore a bit. Fun towns are Hornitos ("little oven", aptly named in the summer), La Grange which was a big hydraulic mining site with enormous rock piles, and Columbia a preserved, restored mining town. Off topic but if you like old mining towns a visit to Bodie on the CA/NV border would be great and give you a different view of CA. Bodie is a
day trip from Yosemite and depending how you go the other towns are not too far out of the way. If you do go to Bodie make sure to find out if the roads are open.