Diamonds, as well as other gems, are weight-measured in carats (ct.). Typically, people think that the size a diamond appears to be is directly correlated with how many carats it weighs. This is not necessarily true as two types of gemstones can have the same weight in carats but be slightly different in physical measurements, such as diamonds and sapphires.
The term Total Carat Weight (t.c.w.) is used when weighing a piece of jewelry that has multiple gemstones on it. Sets of earrings, for example, go by Total Carat Weight because take into account the combined weight of both pieces of jewelry.
When looking at carat weight, it’s important to know that the amount a single diamond weighs is far more valuable than if there are two diamonds with half the weight of the single one. For example, a diamond that weighs 4 ct. is always valued higher than a pair of diamonds that weigh 2 ct. apiece. This is because larger diamonds are far harder to discover in the wild than smaller ones. In fact, diamonds that are 2x heavier in carats can go for nearly 4x more in price!
Finally, there are certain cutoff weights that are widely known in the diamond industry (.50, .75, .90, 1.0, 1.5, and 2.0 ct.). These weights are basically “magic numbers” that diamonds cutters will try to cut their diamonds to, even if it means forfeiting some of the diamond’s overall beauty in the process. Diamonds that come just under these cutoff weights are called Under-Sizes, and can be considered more valuable than standard weighted one simply due to their own type of rareness