A crystalline substance is a substance where the molecules, atoms or ions that compose it are in an extremely regular pattern in three dimensions. This regular pattern is called the crystal's lattice. Whether a substance solidifies as a crystal often depends on the way in which it forms, not its chemical components.
Crystals form from fluids by either cooling to a solid state or depositing from a dissolved state. They can form directly from vapors as well as liquids. As substances crystallize, they can form either a single crystal or a polycrystal. A polycrystal is a single solid object with more than one different crystalline shape or orientation. The final form of crystals depends on the chemistry, pressure and temperature present when they form, and how quickly these factors change during formation.
Crystals can form with a number of different types of chemical bonds between the constituent atoms. Almost all metals exist as polycrystals in a solid state. Covalent bonds are present in crystals such as diamond and quartz. Ionic bonds are present in salt crystals such as table salt. The type of bond that holds a crystal together affects its general properties, with ionic crystals tending to be harder with a higher melting point than covalently-bonded crystals.