A crystalline solid is a type of matter that has its atoms, ions, or molecules arranged in a regular, three-dimensional repeating pattern called a lattice. Having its component units arranged in such a way gives a crystalline solid certain physical characteristics which may vary according to the pattern. All crystals fall into one of seven possible basic shapes. Some of these shapes have more than one possible lattice pattern, of which there are 14 total.
The units of a crystalline solid lattice, whether they are atoms, ions, or molecules, are bonded to each other, giving the solid a strong structure. This regular framework makes crystals particularly resistant to compression. They may be broken or crushed with enough force, of course, but it is very difficult to compress them. Some types of bonds are stronger than others, but all crystals exhibit this property to some degree. The lattice of a crystal is made up of units called cells, the smallest repeating units of the lattice.
The regular lattice framework gives some types of crystals enormous strength. Diamond, well known as the hardest naturally occurring substance, owes its hardness to the way the carbon atoms that form its structure fit together in its crystal lattice. Other common crystalline solids are sugar, rock salt, and some grains of sand like those of quartz. Crystalline solids occur in nature in the form of many types of precious gems and minerals and even some organic materials.
Crystals fall into seven basic shapes. These shapes are rarely found in nature in perfect geometric form, but crystals of a particular substance will always form into its characteristic shape or composites of more than one. Three of the basic crystal shapes are regular, cubic, or right-angle rectangular solids. A fourth form is a hexagonal solid, and the other three are all rectangular solids with some edges meeting at angles other than ninety degrees. Some substances may form composite crystal shapes, but all are combinations of the seven basic crystal shapes.
Within the basic crystal shapes, 14 different lattices are possible. The lattices refer to the particular way the atoms, ions, or molecules are arranged in the crystal, the lattice type affects the shape of the crystal. These lattices also help to determine the physical properties of the crystal such as how the crystal breaks if struck, the way it refracts, or bends, light that passes through it, and its melting point.