In geology terms the difference between igneous and allotriomorph

is that igneous is resulting from, or produced by, the action of great heat; with rocks, it could also mean formed from lava/magma; as, granite and basalt are igneous rocks while allotriomorph is a mineral that did not develop its otherwise typical external crystal form because of late crystallization between earlier formed crystals, typical of matrix minerals in rapidly crystallizing volcanic lavas and shallow igneous intrusions.

As an adjective igneous

is pertaining to, having the nature of fire; containing fire; resembling fire; as, an igneous appearance.

As a noun allotriomorph is

a mineral that did not develop its otherwise typical external crystal form because of late crystallization between earlier formed crystals, typical of matrix minerals in rapidly crystallizing volcanic lavas and shallow igneous intrusions.