For pumice see pi 1670. This adjective deriving from it is variously spelled kishroeidh/s (so LSJ s.v.) and, as here, kisshroeidh/s.
The form given as the present unglossed headword is the neuter nominative/accusative singular, which must be quoted from somewhere. One extant possibility is Theophrastus, Enquiry into Plants 3.7.5, but perhaps a likelier source for the present entry -- albeit in the single sigma version -- is Diogenes of Apollonia as quoted by Stobaeus and Aetius: the moon as a fiery pumice-like mass.