Consider the Different Materials’ Characteristics
Rocks contain a lot of water and so require a long – days or weeks – gentle drying process to avoid steam building up within the rock and breaking the rock.
Rocks and glass have completely different expansion rates and viscosities. Rocks are not consistent as they are made up of different materials. Therefore fusing rocks to glass is not likely to be successful.
Others have suggested using the rocks as a mould, by kiln washing. I don’t think this will be successful either. The best is to make a mould of the rock, then pour investment material into the mould and use that to slump/drape the glass.