What is the difference between a mineral, a crystal and a gem?

Answer: A gem is a stones that is used to make jewelry. A crystal is apure substance that has molecules arranged in "a regular geometric pattern". A gem can also be a crystal, whereas a crystal can not be a gem. Crystals of minerals are classified mainly according to their crystal systems such as cubic, hexagonal, tetragonal, orthorhombic, monoclinic and triclinic, depending on the relationship between the crystal faces and their angles. Each crystal of a particular mineral has its own crytal system.