Abbreviated "ct." and spelled with a "c" is a measure of weight used for gemstones. One carat is equal to 1/5 of a gram (200 milligrams). Stones are measured to the nearest hundredth of a carat. A hundreth of a carat is also called a point. Thus a .10 carat stone can be called either 10 points, or 1/10 of a carat. Small stones like .05, and .10ct are most often referred to by point designations. Note that karat with a "K" is a measure of the purity of a gold alloy. A one carat round diamond of average proportions is approximately 6.5mm in diameter. Note that this relationship of weight and size is different for each family of stones. For example ruby and sapphire are both heavier than diamond (technically, they have a higher specific gravity, so a 1 carat ruby or sapphire is smaller in size than a on carat diamond. See Weights and Measures of Gold, Silver and Precious Gems for more information.
The stone is a unit of weight in the imperial system used informally in the UK and Ireland, almost exclusively as a measure of body weight. Although sanctioned by the EU for use as a supplementary unit it is essentially obsolete outside the UK and Ireland.