The more a diamond weighs, all things being equal, the more it will cost. Weight is different than size. A diamond can look bigger and weigh less, depending on how it’s proportioned.
Diamonds are weighed in terms of points and carats. Carat comes from the carob seed, (from a tree in the Mediterranean region) which was a method of measure in ancient times. There are 142 carats to the ounce!
Because of the density difference between gemstones, two stones can appear to be the same size, yet have different weights. For example: Emerald weighs less than diamond, so an emerald that actually weighs 1 carat will look bigger than a 1 carat diamond. On the other hand, ruby weighs more than a diamond, so a ruby that weighs 1 carat will look smaller than a 1 carat diamond. As discussed previously, this is the result in differences in specific gravity.
• Estimation of diamond weight must also take into account a girdle that is either overly thick or extremely thin. The following adjustments should be made to estimated weights for girdle thickness:
Extremely thin: -1 to –2%
Slightly thick: +1 to +2%
Thick: +3 to +4%
Very thick: +5%
Extremely thick: +7 to +10%
• Spread: the size that diamond appears to be; dimensions can affect price by as much as 10-15%. Dimensions are measured to the nearest 100th. The diameter is measured in two mm dimensions because diamond’s aren’t perfectly round; although there is some latitude, a variance of 0.1 mm is considered out of round and a poor cut.
The more out of round, the lower the price per carat.
Diamonds are priced on a per carat basis; the price jumps disproportionately, however, especially in the larger sizes.
Estimating weight through measurements
Weight in Carats
Diameter in Millimeters Diameter in Inches