A carat (with a Cnot a K) is the unit of measurement used for the weight of gemstones. The word "carat" is derived from the word "carob", which is the dried fruit bean that was used in the times before digital scales to weigh small objects. A carob bean is the largest and lightest object to compare weight to. Back in the day, they could use one carob bean to find the equivalent weight to a one carat diamond.
Here are examples of the carat weight of small melee diamonds:
Here are examples of the carat weight of larger diamonds: