Well, they mostly form in earths crust approximately 3 to 25 miles beneath the Earth's surface. Some Crystals form in the mantle, in very high extreme temperature. Mantle and crustal melts migrate upwards, usually at volcano pipes through the crust, where they are thought to be stored in Magma chambers or trans-crustal, crystal-rich mush zones. Magma is cooling afterward and that is where we mine our beautiful Crystals, but not in their amazing look first.
Quartz Crystal looks like just a stone at the beginning, but when you cut it and open it, Crystal inside is preciously amazingly beautiful. Many colors, structure and form is Magnificent.