Many things are sold based on their weight. Diamonds are no different, and they are measured in metric carats. A carat is divided into 100 points and 1 point is 100th of a carat. It helps to compare carats as dollars and points as pennies.
They are written the same: $1.67 means 1 dollar and 67 pennies. 1.67 carats means 1 carat and 67 points.
Sometimes you might hear us refer to a smaller diamond just by its points. For example, a diamond that weighs 0.10 carats, we might call a 'ten pointer.'
It is important to remember that carat weight doesn't show up the same in every diamond. The same carat weight can look different over the various diamond shapes such as round, pear, princess, oval, cushion, emerald, marquise, radiant, or heart. Also, two diamonds with the same carat weight and shape can fluctuate in size based on how each diamond was cut.
Abbreviations associated with carats:
cts - carats
ct - carat
ctw - carat total weight
When you see ctw, usually this is used when there are multiple diamonds in a finished piece of jewelry. This is where they add all of the weight of each diamond together to get the total.
When all of the other C's are equal, the diamond value increases with carat weight because larger diamonds are scarce and coveted. There are certain weight boundaries called "magic numbers" that the price per carat increases. These sizes correspond with popular weights.
Being popular has its perks, meaning greater demand results in higher value for these weights. Typically magic sizes are in increments of 0.50 ct, with 1.00 carat being the most common.
An example of how the magic number affects the price is if one diamond is 1.47 ct and the other is 1.52 ct, the 0.05 ct difference in size is nearly undetectable. However, let's say the stones were identical in the other 3 C's; the difference in price is considerable.