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 the liquid rock from inside the earth cool and harden. Sometimes crystals form when liquids underground find their way into cracks and slowly deposit minerals. 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 to be clear, but those are the kinds you will usually see in the stores. If you look at the sand under a microscope, you will see that sand is made up of the same minerals as the rocks that the sand came from. When plants start to sprout up in sand, it is turning from being just small bits of rock to being soil. Soil is very important to life on earth. It supports plant life.