Quartz is one of the most abundant mineral crystals and can take many forms. Pure quartz is clear, but when the crystal is formed in the presence of minerals like iron, aluminum, manganese or titanium, atoms from these can substitute for the silicon atoms in the crystal, giving it a tint or color. Crystals can also form within dead, organic material if the material has been buried and minerals have seeped into it. Rather than taking the natural crystal shape, the crystal will form along the lines of the original living material. This result is called a fossil .Examples of this are dinosaurs bones and petrified wood .
Crystals have other value beyond just being pretty.
Many crystals have, but especially quartz, a property called piezoeletricity. If the crystal is cut in the proper way, when it is squeezed it will create a tiny electrical charge along it's edges. One practical use of this is in microphones where quartz crystals are mounted to detect the pressure waves from sound and turn them into electrical signals. The electrical signals can then be sent, recorded, or amplified and turned back into sound by a speaker. The piezoelectrical effect can be reversed so that when an electrical charge is applied to the crystal it will warp as if it were under pressure.