Your question implies that the number is less than one full carat, so you could also write it as .59 carat -- rounding up, meaning that the stone weighs just over half a carat.
If a diamond is a round cut, the mm size at the girdle is about seven if the diamond weighs about 1.25 carats. Other gemstones have different weights relative to their mm measurements.
Your question is not clear. Is it referring to Diamond carat or Gold karat. This is because both have different meaning. While, Carat refers to weight of diamond (or gemstones), Karat refers to...