A crystal is a solid material whose atoms, molecules, or ions are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. Crystals are a special kind of solid material where the molecules fit together in a repeating pattern. Some key features of crystals include:

• Crystal systems: Crystals are classified into one of seven crystal systems: isometric, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, and trigonal. Each is distinguished by the geometric parameters of its unit cell, the arrangement of atoms repeated throughout the solid to form the crystal object we can see and feel.

• Molecular structure: All crystals have an extremely well-organized molecular structure. In a crystal, all of the atoms (or ions) are arranged in a regular grid pattern.

• Appearance: 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.

Crystals can be made up of minerals, like diamonds or emeralds, or other substances like sugar, salt, and ice. Not all solids are crystals, and some objects may appear to be crystals to the naked eye, but outward appearances can be deceiving. For gemologists, the atomic structure of the object is the determining factor.