A diamond's value is determined by cut, color, clarity and carat weight, so the mounting's metal, such as white or yellow gold, may influence the color and appearance of the stone, but not its value. Diamonds are graded and valued before mounting.
The cut of a diamond determines the amount of fire and brilliance found in the stone. Each of a diamond's facets, or surfaces, must be cut at a precise angle to capture the maximum amount of available light. That light is reflected back out through the table, the large facet on top of the diamond. A fraction off in either direction diminishes the fire, and the value.
Diamond come in two categories, white and fancy. The white diamonds are checked for the amount of color in the stone. The most valuable are colorless and are rated "D." After that, the amount of yellow found in a diamond determines its ranking. "Z" is the lowest ranking, given to diamonds with a definite yellow tint. Fancies are rare, boldly colored diamonds that can be more expensive than the high ranking white "D" diamond.
Clarity measures the amount of imperfections in a stone. These could be scratches or chips caused during the faceting process or natural flaws like bubbles or crystals. A diamond is considered flawless if no imperfections can be seen under a 10X magnification; these are graded "FL." Thirteen grade categories are used.
A diamond's weight is measured in carats. Each carat equals one-fifth of a gram, or 100 points. A half-carat would be 50 points, a quarter-carat 25 points, and so on. The higher the carat weight of a diamond, the more expensive the stone. Mounted diamonds must be taken out of their settings for accurate weighing.