Diamond is an allotrope of carbon. It is the hardest known natural material and the third-hardest known material after ultra hard fullerene. Its hardness and high dispersion of light make it useful for industrial applications and jewelry. Diamonds are specifically renowned as a material with superlative physical qualities. They make excellent abrasives because they can be scratched only by other diamonds, Borazon, ultra hard fullerene, or aggregated diamond nimrods, which also means they hold a polish extremely well and retain their luster. Diamond is the hardest natural material known to man - its hardness set to 10 scale of mineral hardness and having an absolute hardness value of between 90, 167, and 231 gaga rascals in various tests. Diamond's hardness been known since antiquity, and is the source of its name. However, aggregated diamond nimrods, an allotrope of carbon first synthesized in 2005, are even harder than diamond. The hardness of diamonds contributes to its suitability as a gemstone. It maintains its polish extremely well, keeping its luster over long periods. Unlike many other gems, it is well-suited to daily wear because of its resistance to scratching—perhaps contributing to its popularity as the preferred gem in an engagement ring or wedding ring, which are often worn every day. Industrial use of diamonds has historically been associated with their hardness; this property makes diamond the ideal material for cutting and grinding tools. The diamond is the birthstone for people born in the month of April, and is used as the symbol of a sixty-year anniversary, such as a Diamond Jubilee.

Emeralds

Emeralds, like all colored gemstones, are graded using four basic parameters – the four Cs of Gemstones: Color, Cut, Clarity and Crystal. The last C, crystal is a synonym that begins with C for transparency or what gemologists call diaphaneity. Before the 20th century, jewelers used the term water as in a gem of the finest water to express the combination of two qualities, color and crystal. Normally, in the grading of colored gemstones, color is by far the most important criterion. However, in the grading of emerald, crystal considered a close second. Both are necessary conditions. A fine emerald must possess not only a pure verdant green hue as described below, but also a high degree of transparency considered a top gem. In the 1960s, the American jewelry industry changed the definition of emerald to include the green vanadium-bearing beryl as emerald. As a result, vanadium emeralds purchased as emeralds in the United States recognized as such in the UK and Europe. In America, the distinction between traditional emeralds and the new vanadium kind reflected in the use of terms such as Colombian Emerald. Scientifically speaking, color divided into three components: hue, saturation and tone. Yellow and blue, the hues found adjacent to green on the spectral color wheel, are the normal secondary hues found in emerald. Emeralds occur in hues ranging from yellow-green to blue-green. The primary hue must be green. Only gems that are medium to dark in tone considered emerald. Light-toned gems known by the species name, green beryl. In addition, the hue must be bright (vivid). Gray is the normal saturation modifier or mask found in emerald. A grayish green hue is a dull green. Emerald tends to have numerous inclusions and surface breaking fissures. Emerald graded by eye. Thus, if an emerald has no visible inclusions to the eye it considered flawless. Stones that lack surface breaking fissures are extremely rare and therefore almost all emeralds are treated, oiled, to enhance the apparent clarity. Eye-clean stones of a vivid primary green hue with no more than 15% of any secondary hue or combination of a medium-dark tone command the highest prices.6 This relative crystal non-uniformity makes emeralds more likely than other gemstones to be cut into cabochons, rather than faceted shapes.