Crystals are minerals that have the chance to grow in the shape that tey were meant to be. Just like your DNA determines the color of your eyes, how tall you will get to be and the shape of your bones, the chemicals that a mineral is made of determines what shape it gets to be. We can tell different minerals apart by what crystal shape they are. Sometimes minerals form in spaces where there is not a lot of room, so they don't have a crystal shape. When there is just big hunk of a mineral, it is called a massive mineral. If there is a definite shape with easy to see flat sides, it is called a mineral crystal.Most of the earth's crystals were formed millions of years ago. Crystals form when th liqued rock fro inside the earth cool and harden. Sometimes crystals form when the liqued rock from inside the earth cool and harden. Sometimes crystals form when liquids underground find their way into crack and slowly deposit minerals. Most mineral crystals take thousands of years to '"grow"" but some like (halite) can form so quickly that you can watch them grow at home! Some people think of crystals as clear pretty rocks that are used for jewelry. Amethyst is a very common quartz crystal. Crystals do not have tobe clear, but those are the kinds you will usually see in the stores.