Editing Crystal (section)

== Polymorphism and allotropy ==
{{Main article|Polymorphism (materials science)|Allotropy}}
The same group of atoms can often solidify in many different ways. [[Polymorphism (materials science)|Polymorphism]] is the ability of a solid to exist in more than one crystal form. For example, water [[ice]] is ordinarily found in the hexagonal form [[Ice Ih|Ice I<sub>h</sub>]], but can also exist as the cubic [[ice Ic|Ice I<sub>c</sub>]], the [[rhombohedral]] [[ice II]], and many other forms. The different polymorphs are usually called different ''[[Phase (matter)|phases]]''.

In addition, the same atoms may be able to form noncrystalline [[Phase (matter)|phases]]. For example, water can also form [[amorphous ice]], while SiO<sub>2</sub> can form both [[fused silica]] (an amorphous glass) and [[quartz]] (a crystal). Likewise, if a substance can form crystals, it can also form polycrystals.

For pure chemical elements, polymorphism is referred to as [[allotropy]]. For example, [[diamond]] and [[graphite]] are two crystalline forms of [[carbon]], while [[amorphous carbon]] is a noncrystalline form. Polymorphs, despite having the same atoms, may have very different properties. For example, diamond is the hardest substance known, while graphite is so soft that it is used as a lubricant. [[Chocolate]] can form six different types of crystals, but only one has the suitable hardness and melting point for candy bars and confections. Polymorphism in [[steel]] is responsible for its ability to be [[heat treating|heat treated]], giving it a wide range of properties.

[[Polyamorphism]] is a similar phenomenon where the same atoms can exist in more than one [[amorphous solid]] form.