Your answer depends on the cut of the stone and what about the stone you are measuring.
Size of diamond is available in different measurements: its mm size across the girdle diagonally, and its carat weight. You can look at the chart below, and depending on the cut, you may be able to estimate the different sizes for your diamond. NB: "Each diamond has its own measurements. Each diamond will differ from the standard millimeter dimension!"
The millimeter size of a diamond of this weight varies, depending on the cut. If the gem is round cut, its measurement at the girdle will be between 16.3 mm (15 carats) and 17.6 mm (20 carats).
The size of these two objects is exactly the same: 5mm.
Diamonds are measured in CARATS. Purity of gold is in KARATS. A 14 carat diamond would be an extremely large diamond- about the size of a marble. Value is based on carat weight, cut, color, and clarity- but that would be a multi-million dollar stone in all likelihood. But I think you are looking at marking of the metal.
The designation .30 carats indicates that the diamond weighs 30/100ths of a carat. The mm size will depend on its 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.
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.
Millimeter is a size measurement; carat is a weight measurement. Take the stone to a local jeweler and ask for its weight.
A two carat round brilliant cut stone will measure about 8.2 mm. A different cut will have a different measurement.
A carat diamonds size is 3.5mm.
Size of diamond is available in different measurements: its mm size across the girdle diagonally, and its carat weight. You can look at the chart below, and depending on the cut, you may be able to estimate the different sizes for your diamond. NB: "Each diamond has its own measurements. Each diamond will differ from the standard millimeter dimension!"
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.
The millimeter size of a diamond of this weight varies, depending on the cut. If the gem is round cut, its measurement at the girdle will be between 16.3 mm (15 carats) and 17.6 mm (20 carats).
These gemstones weigh the same: one carat.
One carat equals 100 in this example. If the diamonds are of equal size, then each diamond weighs 4.5454545 carats.
A diamond of this size is unknown, and therefore, not something that can be valued.