A unit of mass, commonly used to measure gemstones.
Technically it measures weight, though it’s often used to refer to size. However, weight and size increase together, so it’s roughly the same thing.
It’s 200mg or a very small fraction of an ounce (less than 1/100). It was standardized in 1907.
Here’s where the word comes from:
Now, the word carat is actually derived from the word “Carob” which is a specific bean of the locust tree. Since Carob seeds were believed to be very even in their weight distribution, they were used to measure fine jewelry in the past.
Several resources noted that the consistency of carob seed weight is actually a myth.
This page shows a nice visual comparison of how big diamonds of various carats are. This page at a diamond wholesalers shows a graph for 1ct prices, which range considerably based on quality, but they average $4,000. This page listed roughly the same valuation.
GemVal is a gemstone evaluation company and they appear to publish their pricing charts for all different kids of gemstones.
Important point: “karat” is an entirely different word that refers to the fineness of gold (nothing to do with weight or mass). Gold is often mixed with other metals. One karat is a mass which is 1/24 gold, so “24-karat gold” is pure gold, and nothing else.
Here’s an entire page explaining how the two usages are different, yet frequently confused.