Found in Southern California thees are on a lawn chair to help give u an idea how big they are !
(UPDATE) SOMEONE STOL THE SAFE I WAS KEEPING IT IN LOL OUT OF BARSTOW CALIFORNIA
They appear to be "glass slag," a byproduct of glass manufacturing. Chunks of glass-slag have been sold as decorative Landscaping stones at the Lawn-&-Garden section at Home Depot and Lowes, etc. They do make a pretty conversation-piece. That is also why I've seen them used as "decorative" stones in aquariums.