adjective, slag·gi·er, slag·gi·est.
Examples from the Web for slaggy
It was hard walking over the shingly, slaggy stones, but they made their way to the summit at last.A Desert Drama|A. Conan Doyle
Slaggy and scoriaceous phases are common, but probably represent only a small part of the different flows.
In a basal flow in Moraine Park, the slaggy and compact phases show differences in phenocrysts as well as in groundmass.