Garnets, gold and graphite are all examples of crystalline solids. These items are all defined as crystalline solids by a similar molecular structure characterized by a well-defined, regular arrangement of identical unit cells.
If a curious student were to look at a crystalline solid, a diamond, for example, under an extremely powerful microscope capable of revealing the diamond's atomic structure, she would see repeating patterns. These repeating patterns are what differentiates crystalline solids from amorphous solids such as glass, which have a much more disordered atomic structure. In addition to gemstones such as diamonds and garnets, metals, table salt and sugar crystals are all examples of crystalline solids.