The diamond is among the most prized and most beautiful gemstones in jewelry. It is the hardest of the naturally occurring minerals, and is usually cut into a brilliant shape. It is also widely used for industrial purposes. The classic brilliant-cut diamond has 57 facets. This provides the beautiful and unique refraction of light that makes it so valuable. The value of processed diamonds is determined by 4 criteria - weight, color, form of processing and clarity. Depending on these criteria a diamond can cost millions. The diamond is a mineral, a natural polymorphic allotrope of carbon. It is so called because of its high hardness and resistance to corrosion. It has very high chemical and physical resistance - it does not dissolve in acids and bases and practically does not wear out. Diamond has several unique qualities unmatched in nature by any other mineral. It is the hardest naturally occurring substance, the best conductor of heat, has the highest melting point and the highest refractive index of all minerals.