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.
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The cost of Platinum diamond earrings will vary greatly depending on the carat weight of the diamond. As an example for prices, 1/2 carat total weight start at about $1400 but a 1 carat total weight sells for approximately $3600 per pair.
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.
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.
A two-carat diamond is a gemstone that weighs two carats.
One carat weight of diamond weighs .20 grammes, so your diamond is about half that weight, or roughly half a carat.
A 1.00 point diamond is one carat, so a .12 point diamond is a little over 1/10th of 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.
The diamond weighs .125 carats. Its mm measurements depend on the cut.
An eighth of a diamond carat, if it is a round cut, measures about 3.2mm at the girdle.
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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.