Diamonds are the hardest substance on earth. They are more brilliant than any other natural gem. Their sparkling fire, durability, and rarity make them the most prized of gems.  The most popular diamonds are colorless. However, most diamonds have a light tint, usually yellowish or brownish. Perfectly clear diamonds are much more valuable than tinted diamonds. Very rarely, diamond occurs in deep hues of red, blue, and green. Such diamonds, known as "fancies", are extremely valuable.

    Diamonds are the most lustrous of true gems. They have a brilliant luster, and also exhibit dazzling color flashes known as "fire". This is caused by dispersion, where light enters the diamond and splits into the colors of the spectrum.  Only 20 percent of mined diamonds are used in jewelry, as most are unsuitable. A flawless diamond is exceptional; even diamonds used as gems contain flaws. A grading system, instituted by the GIA (Gemological Institution of America), was implemented to evaluate diamonds based on their clarity. Three additional evaluation systems were devised by the GIA.

    Diamond is the most important gemstone in the industry. Roughly 80 percent of the gem trade is limited to diamonds alone. Diamonds of all different colors are faceted into various cuts; the colorless variety is most in demand. The other colors are less commonly worn in jewelry. Deep red, green, and blue diamonds are extremely rare and highly prized. Black, opaque diamond (bort) is occasionally faceted into a black gemstone with a metallic luster.