Originally Posted By: Jackie
I looked up the origin of igneous, and found:
igneous
1664, from L. igneus "of fire, fiery," from ignis "fire," from PIE *egni- (cf. Skt. agnih "fire, sacrificial fire," O.C.S. ogni, Lith. ugnis "fire").
the "other" OED
I don't know what O.C.S. is, but that seems to verify Faldage's idea.
I feared there would be no clear system, as the answer to the question took a while. From what you show here I would hesistate about choosing "ignious" . Or is it best to look at the most recent origin from L.igneus?

I will never learn all those by heart and so am condemned to keep
checking the books. \:\(