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Crystal
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<p>A crystal or crystalline solid is a <a href="/facts/Solid/hj5UsXno">solid</a> material whose constituents (such as <a href="/facts/Atom/FoQ4kGN5">atoms</a>, <a href="/facts/Molecule/N9ljSfFq">molecules</a>, or <a href="/facts/Ion/3bePpDna">ions</a>) are arranged in a highly ordered microscopic structure, forming a <a href="/facts/Crystal_lattice/7gQKTsXk">crystal lattice</a> that extends in all directions. In addition, macroscopic <a href="/facts/Single_crystal/UMG5pFp9">single crystals</a> are usually identifiable by their <a href="/facts/Geometry/wTQf5ffQ">geometrical shape</a>, consisting of flat <a href="/facts/Face_(geometry)/T2E6BTU4">faces</a> with specific, characteristic orientations. The scientific study of crystals and crystal formation is known as <a href="/facts/Crystallography/9B5toeqG">crystallography</a>. The process of crystal formation via mechanisms of <a href="/facts/Crystal_growth/QwhSDA5y">crystal growth</a> is called <a href="/facts/Crystallization/oCUUG80k">crystallization</a> or <a href="/facts/Solidification/0kNr25SF">solidification</a>. </p><p>The word <i>crystal</i> derives from the <a href="/facts/Ancient_Greek/LIK7NlUh">Ancient Greek</a> word κρύσταλλος (<i><i>krustallos</i></i>), meaning both "<a href="/facts/Ice/BGvkKFjD">ice</a>" and "<a href="/facts/Quartz/4n86tE2G">rock crystal</a>", from κρύος (<i><i>kruos</i></i>), "icy cold, frost". </p><p>Examples of large crystals include <a href="/facts/Snowflake/KUWqwqbP">snowflakes</a>, <a href="/facts/Diamond/IBUdnd1y">diamonds</a>, and <a href="/facts/Table_salt/SCplalQ3">table salt</a>. Most inorganic solids are not crystals but <a href="/facts/Polycrystal/nXSgR0wp">polycrystals</a>, i.e. many microscopic crystals fused together into a single solid. Polycrystals include most <a href="/facts/Metals/PT8hNX6R">metals</a>, rocks, <a href="/facts/Ceramics/9JKudpqX">ceramics</a>, and <a href="/facts/Ice/BGvkKFjD">ice</a>. A third category of solids is <a href="/facts/Amorphous_solid/YTIzmrR0">amorphous solids</a>, where the atoms have no periodic structure whatsoever. Examples of amorphous solids include <a href="/facts/Glass/hezRsAGf">glass</a>, <a href="/facts/Wax/MAXhjzNh">wax</a>, and many <a href="/facts/Plastic/Kla5QItn">plastics</a>. </p><p>Despite the name, <a href="/facts/Lead_glass/FtgPe6r2">lead crystal, crystal glass</a>, and related products are <i>not</i> crystals, but rather types of glass, i.e. amorphous solids. </p><p>Crystals, or crystalline solids, are often used in <a href="/facts/Pseudoscientific/kdNzoLeS">pseudoscientific</a> practices such as <a href="/facts/Crystal_therapy/fgo8IE5Q">crystal therapy</a>, and, along with <a href="/facts/Gemstone/kHffmvE3">gemstones</a>, are sometimes associated with <a href="/facts/Spell_(paranormal)/9fgfupQz">spellwork</a> in <a href="/facts/Wicca/q623sLWu">Wiccan</a> beliefs and related religious movements. </p>