A Carat is an unit of weight for diamonds and other gemstones. One Carat is equal to 200 milligrams. In other words, one pound equals 453 grams, or 453,000 milligrams. So, a person weighing 170 pounds would weigh 385,050-carats. CT is the common abbreviation used by the jewelry industry.
On the other hand a Karat is an ancient unit used to measure the purity of gold. 24 Karat refers to pure gold, which is not often used in jewelry because it is very soft. Gold is usually mixed with other metals to make it harder or modify its color.
If a piece of jewelry is made of metal that is 22 parts gold and 2 parts copper, that is 22-Karat Gold. Similarly if jewelry is of 18-Karat Gold that means it is 18 parts gold and 6 parts copper.