A crystal is a hard, solid substance made of molecules that bond together in specific patterns to form an interesting shape that has straight edges and flat surfaces. Not all crystals have the same shape, as you can see in the salt vs. sugar project - there are actually lots of different kinds of crystals, and each kind has its own special shape. Some crystals that you are probably very familiar with are sugar, salt, and ice. But many other solids are made of crystals too; we just can't see them because they are so small! Lots of minerals form beautiful crystals that are used for jewelry, like diamonds or emeralds. Crystals are often transparent, which means that you can see through them sort of like you can see through glass. Other kinds have beautiful colors.