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. The word "crystal" comes from the Ancient Greek word "krustallos," meaning both "ice" and "rock crystal," from "kruos," meaning "icy cold, frost". Crystals are built out of atoms, and the fundamental building blocks for atoms are protons, neutrons, and electrons. The scientific definition of a crystal is based on the microscopic arrangement of atoms inside it, called the crystal structure. A crystal is a solid where the atoms form a periodic arrangement.

Key points about 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.

• Crystal structure: A crystal structure (an arrangement of atoms in a crystal) is characterized by its unit cell, a small imaginary box containing one or more atoms in a specific spatial arrangement. The unit cells are stacked in three-dimensional space to form the crystal.

• Amorphous solids: Not all objects with regular geometric faces are crystals, nor are all solid materials crystals. Glass, for example, has a non-crystalline, amorphous atomic structure. Although glassmakers can pour and harden glass into geometric shapes, its atomic structure remains unchanged.

• Types of crystals: Crystals can be made up of different molecular compositions, forming structures of different shapes. For example, table salt naturally forms cubic crystals, while quartz crystals, made up of silicon dioxide molecules, form hexagonal prisms with hexagonal pyramids on either end. Snow is a form of ice crystal, made up of water molecules that form flat hexagonal plane crystals.

• Uses of crystals: Crystals have a wide range of uses, from industrial applications to pseudoscientific practices such as crystal therapy. Well-known gems that are crystals include diamonds, garnets, and spinels.