That's what I was kinda thinking at first. The stone on the left weighs 1 lb 4.8 oz & measures 3 3/4 x 5 and the one on the right weighs 1 lb 11.2 oz & measures 5 x 3. They are rock solid and do have some dirt embedded into the groves so I'm thinking they are some type of stone. I found these at a garage sale and paid a couple of bucks for them.
I feel very confident that it is slag glass, more than likely used in manufacturing marbles, although I have seen paperweights and other stuff made from similar material. (You might try posting this in the marble forum, they may be able to tell you more about it) More than likely vintage glass, maybe even antique. Is there any history of a glass factory near where you got these? Neat stuff for sure!