You might remember this one from earth science class. An igneous rock is one that forms through intense, fiery heat––usually in a volcano. It starts out molten (so hot it melts into liquid), then solidifies as it cools. So, it’s rock that has “ignited.”
Igneous comes from the Latin ignis "fire." Granite and basalt are good examples of igneous rock that started out as blazing hot lava and morphed into harder stuff as their temperature dropped. FYI, the other main types of rocks are sedimentary and metamorphic and, technically, you could use igneous to describe other things that are rare or fiery, but it would sound pretty stilted and most people wouldn’t know what you meant.