based on the grainy texture of the outside of it, my guess is that's it's most likely just colored glass that's spent a long time in a sandy/gritty soil
I agree, it is worn glass which naturally won't be affected by a hot pin. One can only see inclusions if the item in question is clear, it cannot have a worn surface as does the item in question were it to be a synthetic stone. Garnet does wear poorly, but it look like glass to me.