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.