The word comes from the Greek Igne, meaning fire. The term 'igneous' means derived from heat or fire. While its most common modern usage is regarding geology, the term can be used as a basic description of the origins of a subject material or as metaphor for a fiery situation. The geological usage refers to a class of materials, not just 'rocks'. Igneous materials in geology can include crystals, various sulfur compounds, and related forms.