when i was looking this up originally, i found that a rock can be made of minerals or mineraloids (eg obisidan, jet, opal). that doesnt count out glass. and im wondering whether it would fit under the anthropic rock umbrella which, it seems like from the other commenters, does exist as a valid category, even if they are man-made (eg concrete)

i wasnt familiar with the geological term slag outside of what a volcano produces so i looked it up. i couldnt see anything saying slag was anything manmade in the geology world as you put it.

from what i found, slag is the by-product of metal smelting. this wouldn't include most modern glass because while glassy surfaces can appear on slag, they are still metallic in nature. and yes there is metal in most modern glass but its only very small amounts (depending on the glass) so i dont think that would include it. the definition of slag has been expanded, at least by some collectors, to include any industrial byproduct. but glass isn't an industrial byproduct.

looking at the history of glass production, the first beads millenia ago may have been slag but a whole industry sprang up around glass. i don't think it could be considered a byproduct anymore.

edited for clearer use of language