The weight of a diamond is usually expressed in carats. The term carat originated in ancient times when gemstones were weighted compared to the carob bean, where one bean weighed about one carat. The term was standardized and converted to the metric system in 1913 so that the current carat equals 0.2 grams, which is a little more than 0.007 ounce.

Knowing the technical definition of a carat is one thing, but the diamond shopper needs to understand carat weight so they can make the best purchase decision for their diamond. The carat weight of a diamond influences several key factors that should be part of the diamond decision.

Sometimes in the jewelry trade, the term carat weight is used synonymously with size. The implication is that all diamonds that weight one carat are the same size and those that weight two carats are the same size. This is not accurate and an astute diamond shopper will work to understand the difference between size and weight.

The size of diamonds can vary with the same weight because the shape of the diamond can be different. A one carat round diamond has a very different length and width than marquise shaped diamond weighing one carat.

The diamond industry has developed its pricing structure based on the carat weights that consumers request when they go into a jewelry store. Even carat weights have taken on a social importance far above their simple weight so consumers typically ask for whole carat weights or simple fractions of a carat. For example, a diamond shopper might ask for a half-carat, one carat, a carat and a half, or a two-carat so the diamond industry sets the price increases at these weights. A diamond weighing 1.00 carats has a significant higher price per carat than one that weighs 0.99 carats so diamond cutters to get the finished diamond weight at or over these requested carat weights. As a result, there are many more diamonds cut just over 1.00 carat than just under. Knowledgeable shoppers know there are some great values at just under the even carat weights but with few diamonds cut, the demand is greater than the supply.

The weight also affects the price in that the larger the carat weight, the fewer diamonds are available. Diamonds loose approximately 40-60% of their rough weight in the cutting and polishing process. Therefore, it can take over a two-carat rough diamond to produce a 1.00 carat finished diamond. Most diamonds mined are very small with larger stones relative rare. It can take many thousands of rough diamonds mined to yield one that results in a 1.00 carat finished diamond. The price of larger carat weights increases with the rarity. For example, a two-carat diamond can be almost four times the price of a one-carat diamond with the same quality.

Even with the same shape of diamond, the carat weight does not tell you the size. It is similar to asking how tall a 200-pound man is. Just as the height of a man can vary with the same weight, a diamond's length and width can vary for the same carat weight. The challenge is to find diamonds that have the right depth parameters to give them beautiful brilliance and sparkle in addition to big size for the carat weight.

The weight of a diamond also affects the size, as diamonds get heavier the length and width do not go up as fast as the weight. For example, a two carat round diamond weighs 100% more than a one-carat diamond but the surface area you see only increases 64%.

Note that as the carat weight goes up, the price per carat goes up faster and the size goes up slower. The result is that as the carat weight goes up it takes a lot more weight and even more money to make the diamond noticeably bigger to the eye.

Perhaps the biggest question a diamond shopper has is what carat weight of diamond they should purchase. Of course, their budget is going to be limiting factor in determining what carat weight to buy but the target carat weight is influenced by several factors including the size of the finger, type of mounting, and the carat weight of friends and family diamonds.

Buying the right diamond requires consideration of many factors and understanding the impact of carat weight is one of the keys to making the best purchase decision possible.

Denny Reinke is the Vice-President of Diamond Source of Virginia, an online diamond retailer specializing in loose diamonds, diamond rings and diamond jewelry located in Richmond, Virginia and on the web at www.DiamondSourceVA.com You can also visit Denny's blog at www.diamonds.blogs.com