Carat (ct.) refers to the unique unit of weight measurement used exclusively to weigh gems and diamonds. Carat weight is often confused with visual size even though it is actually a measurement of weight. Depending on the shape and type of gemstone being weighed, the weight will visually show itself differently. For example, a 1.00 ct. round diamond will measure around 6.5mm, and a 1.00 ct. round sapphire will measure around 6.0mm. This is due to the varying density of different gemstones.
Once cut, color, and clarity grade have been determined, the carat weight of a diamond can be easily established to fit within a budget. Larger diamonds are much more valuable because they are discovered in nature much less frequently than small ones. Diamond prices actually rise exponentially with carat weight rather than linearly. For example, a 1.00 ct. diamond of a given quality is always valued higher than two 0.50 ct. diamonds of the same quality. In fact, a general rule of thumb is that a diamond of double the weight costs around four times more.
"Under-sizes" are diamonds that weigh just below a cutoff weight. While fewer exist, they may represent an enhanced value. They are more difficult to find as a diamond cutter will choose to sacrifice beauty in order to cut a diamond with a weight that reaches one of the cutoff weights, or "magic numbers" as they are known in the industry. The cutoff weights are 0.50 ct., 0.75 ct., 0.90 ct., 1.00 ct., 1.50 ct., and 2.00 ct.
Total carat weight (t.c.w.) is a phrase that represents the total weight of all diamonds or other gemstones in a piece of jewelry, when more than one gemstone is used. Diamond solitaire earrings, for example, are usually quoted in t.c.w., indicating the combined weight of the diamonds in both earrings.