Jewelers at the Antwerp World Diamond Exchange recommend spending two months' salary to buy a diamond engagement ring. Obviously, what you get will depend on how much you make. But jewelers base a diamond's value on its cut, clarity, color and carat. A carat is a measurement of a diamond's weight, and the diamond's cut will determine the size that corresponds to the carat weight.
The cut of a diamond does not refer only to its shape, but also to "the metric used to evaluate the precision and quality of the overall cutting itself," according to DiamondInfo.org. Cut is the most complex factor in determining a diamond's worth. Jewelers examine the proportions, symmetry and polish of a diamond's cut. A properly cut diamond reflects light from facet to facet before dispersing it out the top, or crown, of the gem.
Diamonds are made solely of carbon, so any materials in a finished diamond are considered imperfections, called inclusions. Often they are visible only through a jeweler's loupe, which magnifies the diamond by a factor of ten. Flawless diamonds -- with no inclusions -- are rare and valuable. Flaws interfere with the movement of light through the stone, reducing its brilliance. Diamonds are the hardest natural substance on Earth -- only another diamond can cut a diamond--and flaws also reduce a stone's durability.
For diamond color, a grade of D, E or F is the most desirable because it denotes a colorless stone, sometimes called a white diamond. The farther you go in the alphabet, the more a diamond tends to yellow. G through J are good quality stones, nearly colorless. Stones rated K through M aren't recommended for large cuts, especially engagement rings, because of their noticeable discoloration. Along with cut, color is the most important factor in choosing a diamond.
A carat is a form of diamond measurement that's divided into 100 points; a diamond with 75 carat points is .75 carats. One-carat diamonds will have different sizes, depending on the shape. A round, one-carat diamond is 6.5 millimeters in diameter. A princess shape is 5.5 mm square. The sizes for one-carat diamonds in other cuts are: emerald (7 by 5 mm), pear (8 by 5.5 mm), oval (7.5 by 5.5 mm), heart (6.5 by 6.5 mm), trilliant (6.5 by 6.5 mm), marquise (10 by 4.75 mm), radiant (7 by 5 mm), Asscher (5.5 by 5.5 mm) and cushion (5.5 by 5.5 mm).