It's all about the 4 Cs
'Carat' refers to the weight of a diamond. Each carat is divided into one hundred points and is equivalent to 0.2 grams, a measurement adopted in the United States in 1912 and now universal. The name comes from the carob seed, used by early traders as near-uniform counterweights to balance their scales.
'Clarity' indicates the presence (or lack of) inclusions inside the diamond, as well as external marks or blemishes. Most diamonds contain minute inner flaws or inclusions, often referred to as 'birthmarks', as they occurred during the long formation process of the diamond, over millions or billions of years under extreme heat and pressure deep within the earth's core. Clarity is graded according to the visibility of inclusions or blemishes under tenfold magnification. The top grade, reserved for the rarest and most highly prized diamonds, is flawless (FL), showing no inclusions or blemishes. This ranges downward through IF, internally flawless, meaning only blemishes and no inclusions, difficult to see under magnification even for an experienced grader; VS, or very slight; SI, slight; down to I, obvious inclusions which ultimately affect brilliance.
In terms of 'Color', the finest white diamonds (as opposed to fancy colored diamonds) should be as close to colorless as possible. Color grading as defined by the GIA is classified by letters of the alphabet, the best being D, colorless, and continuing through G to J, near colorless, right down to Z. Color gradations for each letter are subtle but clearly defined to the professional eye. The letter D was chosen to represent the top white diamond color, as other pre-existing, imprecise and confusing classifications had used letters A, B, and C among other labels.
'Cut' is the most crucial of the 4 Cs as it not only governs shape but affects and influences the Fire, Life and Brilliance of a diamond. Brilliance often refers to the light overall light reflected from a diamond; Fire is the dispersion of light into the rainbow colors of the spectrum; and Life is the movement of light within a diamond, or scintillation. Aside from the shape of the diamond, from the classic round brilliant, to fancy cute, a cut is evaluated on the precision and its critical angles, its symmetry, proportions and polish. Proportions concern the relationships between different parts of the diamond: the table (top flat facet), the crown (visible top of the diamond), the girdle (the line or edge around the middle of the diamond), and the pavilion (the underside). These relationships can affect the way the diamond interacts with light.