Lava Beds National Monument

This is a picture from Lava Beds National Monument. There are many caves there formed by cooling lava flows that retreated into the Earth. In most of them, the lava cooled quickly which led to brittle and rough walls, but in this one the lava cooled slowly. As a result the walls are smooth and minerals are baked into them. The minerals make it look like colored glass.
This monument is in California very close to the state line with Oregon.