In some natural marble samples, you can see all kinds of fossils like ammonites and orthoceras, natural history in a slab. Bio-Glass is sort of a modern green version; it is made from 100% recycled glass squished together and sliced into slabs, and you can sometimes see the bottoms of wine bottles, the fossilized remnants of our modern waste.
Each colour and style of bioglass represents a different material stream; one whiteish one comes from plate glass, another from drinking cups. Old beer bottles are turned into a lovely brown bioglass. it is a shame that their website is not so explicit about the sources; it is an interesting story.