What's the main difference between a crystal and quartz?

Native speaker’s answer
Rebecca
The main difference is that a "quartz" is a type of stone, whereas a "crystal" is the geometrical shape of a stone. One can get crystal quartz, crystal agate, or crystal opal. They'd all be geometrically flat and have symmetrical planes on them. Ex: I need to add a few crystals to my collection. I already have quartz. Ex: I really like how crystal quartz looks.