It looks like terra cotta. It doesn't seem to be as hard, though. When I installed (or tried to install) water heater straps, the toggle bolts pulled out of the brick - too soft. I should add that the foundation is in good shape -- no cracks, etc.
I own a home with a foundation made of a red clay brick (a little smaller than cinder block, but similar design with vertical marks on the face). They are not concrete filled. A seismic engineer who looked at it several years ago said that it was called firebrick and was commonly used to house...
