The 4 C's
From the GIA (Gemological Institute of America)
The key to a diamond’s value is its rarity, and no two diamonds are alike. Rarity is determined by a diamond’s unique characteristics as measured by the 4 C's: Cut, Color, Clarity and Carat. Using these criteria, a small diamond of exceptional quality will likely be more valuable than a larger diamond of lower quality.
Clarity refers to a stone’s relative position on a flawless-to-imperfect scale. Clarity characteristics are classified as inclusions (internal) or blemishes (external). The size, number, position, nature and color or relief of these characteristics determines the clarity grade. Very few diamonds are flawless, showing no inclusions or blemishes under 10x magnification. The GIA Clarity Scale includes eleven grades ranging from Flawless to I3. If other factors are equal, flawless stones are most valuable.
Grading color in the normal range involves deciding how closely a stone’s bodycolor approaches colorlessness. Most diamonds have at least a trace of yellow or brown in their bodycolor. With the exception of some natural fancy colors, such as blue, pink, purple or red, the colorless grade is the most valuable. The GIA Diamond Grading System uses letters to represent colors, beginning with D (colorless) and ending at Z (light yellow or brown).
The proportions and finish of a polished diamond are its cut, or make. Cut can also mean shape, as in round cut or princess cut. Proportions are the size and angle relationships between the facets and different parts of the stone. Finish includes polish and details of facet shape and placement. Cut affects both the weight yield from rough and the optical efficiency of the polished stone; the more successful the cutter is in balancing these considerations, the more valuable the stone will be. GIA provides a cut quality grade for standard round brilliant diamonds that fall in the GIA D-to-Z color range. The GIA Cut Scale ranges from Excellent to Poor.
The metric carat, which equals 0.200 gram, is the standard unit of weight for diamonds and most other gems. A carat weighs about the same as a small paper clip. Just as a dollar is divided into 100 pennies, a carat is divided into 100 “points.” This means that a diamond of 50 points weighs 0.50 carats. But two diamonds of equal weight can have very different values depending on their clarity, color and cut.
What is a Diamond?
A diamond is a mineral composed essentially of carbon crystallized at extremely high temperatures and pressures; in nature, diamonds form 150 to 200 kilometers (93 to 124 miles) or more below the earth’s surface. Diamond is the hardest of all known natural substances (10 on the Mohs scale). Its refractive index is 2.417, dispersion 0.044, specific gravity 3.52, and its luster is adamantine. Diamond forms in the cubic, or isometric, crystal system, has four directions of perfect octahedral cleavage, and shows a step-like fracture surface. Its color ranges from colorless to yellow, brown, gray, orange, green, blue, white, black, purple, pink and (extremely rarely) red.