Do all igneous rocks have crystals?
Sally C’s answer is good. -Most- igneous rocks are made up of crystals, though the crystals may sometimes be too small to see even with a hand lens. Sometimes we have to slice the rock int very thin slices, and examine it with a petrological microscope to see the crystals. (this is quite fun, especially as under polarised light you get fantastic colour effects when you look at many thin sections. Also, because the crystals grow close to one-another then they often do not have the space to develop the nice regular shapes we associate with crystals. The group of igneous rocks that are not made up of crystals are volcanic glasses (obsidian is a well-known example) – the material cools so fast it does not arrange the atoms into an neat regular structure (as in a crystal) (Edit)- one groupof igneous rocks thatcan cause a bit of aa problem in the field are volcanic tuffs.
