A diamond is valued by its cut, clarity, carat weight and colour. A local jeweler can show you stones in this category.
A two carat round brilliant cut stone will measure about 8.2 mm. A different cut will have a different measurement.
Today on Blue Nile, you can purchase a one carat SI1 diamond and pay about US$4,759; and a one carat VVS1 diamond and pay about US$12,252.
Most people think carat is a size. Actually, it is not, it is a weight. Diamonds are measured in Carats. Carats are a metric weight unit with one equaling about the weight of a paper clip or .2 grams. The GIA website has a lot of information on different carat weight.
There are about 1.092 x 10^21 carbon atoms in 2 grams of 1 carat diamond. This is calculated based on the molar mass of carbon and Avogadro's number.
The weight of a diamond is measured in carats, not in cents. So, a 30 cent diamond would not be equivalent to any specific weight in carats. To determine the carat weight of a diamond, it would need to be weighed using a scale calibrated to measure carats.
A two-carat diamond is a gemstone that weighs two carats.
A 1.00 point diamond is one carat, so a .12 point diamond is a little over 1/10th of a carat.
One carat weight of diamond weighs .20 grammes, so your diamond is about half that weight, or roughly half a carat.
A one carat diamond in the round brilliant shape measures aproximately 6.5mm at the girdle. If the diamond is another cut, measurements will be different.
You've given the carat weight of a diamond. 'Big', then, is based on the cut of the stone.
Carat is a weight, so its weight is one tenth of a carat or one tenth of .20 grams.
A 2 mm round diamond weighs about .03 carats, so a 1.5 mm round diamond would be smaller than .03 carats. A .01 carat diamond is about as big as a grain of sand.
A 2 pt diamond is a diamond weighing .02 carats or 2/100ths of a carat.
A two carat round brilliant cut stone will measure about 8.2 mm. A different cut will have a different measurement.
An eighth of a diamond carat, if it is a round cut, measures about 3.2mm at the girdle.
The diamond weighs .125 carats. Its mm measurements depend on the cut.
If the diamond is an ideal emerald cut with these measurements, the diamond may weigh about one quarter of a carat. Otherwise, you have stated the diamond's 'big' size in measurements.