Crystalline refers to a solid material whose constituents, such as atoms, molecules, or ions, are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. Key features of crystalline solids include:

• Periodic translational ordering: The atoms or molecules form a three-dimensional arrangement within a single repeating unit called a unit cell. The unit cell structure repeats in all directions at regular spacing, filling a regular three-dimensional grid called a lattice.

• Distinctive internal structures: Crystalline solids have regular ordered arrays of components held together by uniform intermolecular forces, whereas amorphous solids do not/12%3A_Solids/12.01%3A_Crystalline_and_Amorphous_Solids).

• Examples: Examples of large crystals include snowflakes, diamonds, and table salt. Most inorganic solids are not crystals but polycrystals, i.e. many microscopic crystals fused together into a single solid. Polycrystals include most metals, rocks, ceramics, and ice.

Crystalline solids can be of different types, depending on the nature of bonding between unit cells within the lattice, such as metallic, ionic, network (extended covalent), and molecular. Although crystals are often associated with transparency, non-crystalline (amorphous) solids such as glass can also be crystalline.