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.
A carat diamonds size is 3.5mm.
Carat is not a measurement of size, its weight. 200 miligrams=1 carat
These gemstones weigh the same: one carat.
If 7.7 is the carat weight, then the size is 7.7 carats. The millimeter measurements of a diamond with this weight will vary, depending on the cut.
The carat size of a diamond should be determined by a certified jeweler. There are two different sizes the is the millimeter size which as stated was 7mm and then a jeweler can give a carat size.
each carat is 200 miligrams
carat
A two carat round brilliant cut stone will measure about 8.2 mm. A different cut will have a different measurement.
It can be resized.
One carat equals 100 in this example. If the diamonds are of equal size, then each diamond weighs 4.5454545 carats.
The diamond weighs 40/100ths of a carat. Other measurements depend on the style of its cut.
Carat is a weight measurement for a diamond: here size matters -- bigger is better, biggest is best.