If the diamond is a round brilliant cut, the 3mm measurement at the girdle indicates that the diamond weighs about 1/10th of a carat, or .10 carats.
If the diamond is a round brilliant cut, this measurement at the girdle indicates that it could weigh between .20 carats (3.8 mm) and .25 carats (4.1 mm). If the diamond is of a different cut, a jeweler can remove the diamond and give you the exact carat weight.
If the diamond is a round brilliant cut, the measurement across the girdle is between 5.9 mm, which weighs approximately .75 carats, and 6.2 mm, which weighs approximately .85 carats. If the diamond is a different cut, there is not enough information in your question to answer it: other cuts require more than one measurement.
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.
Forty-five carats is large for any diamond. For reference, a five carat princess cut diamond measures 9.5 x 9.5 mm.
If the diamond is a round brilliant cut, the 3mm measurement at the girdle indicates that the diamond weighs about 1/10th of a carat, or .10 carats.
If the diamond is a round brilliant cut, this measurement at the girdle indicates that it could weigh between .20 carats (3.8 mm) and .25 carats (4.1 mm). If the diamond is of a different cut, a jeweler can remove the diamond and give you the exact carat weight.
If the diamond is a round brilliant cut, the measurement across the girdle is between 5.9 mm, which weighs approximately .75 carats, and 6.2 mm, which weighs approximately .85 carats. If the diamond is a different cut, there is not enough information in your question to answer it: other cuts require more than one measurement.
A round cut diamond, cut to industry standards, that is 5.1 mm across the face of the diamond (the top) weighs half a carat.
If the diamond measures 7.5 x 7.5 mm, the diamond may weigh about two carats. You can ask a certified gemologist to weigh the diamond for an exact carat weight.
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.
The mm size of a diamond stone relative to its weight in carats is directly related to the cut and the excellence of the cut. For example, a round brilliant stone measuring 8 mm in diameter at the girdle weighs about two carats. A different cut will have a different weight in carats.
A round, brilliant cut diamond measuring 6.5 mm at the girdle weighs about one carat.
A diamond weighing .07 carats (7/100ths of a carat) measures about 2.7 mm if the diamond is cut in the round brilliant style.
Forty-five carats is large for any diamond. For reference, a five carat princess cut diamond measures 9.5 x 9.5 mm.
Carats indicates weight; mm indicates length and width -- the two are not necessarily compatible. However, if your diamond is a round brilliant cut diamond and its diameter at the girdle is 10mm, the diamond may be about four carats. If the cut is another shape, it may have a different carat weight.
The diamond weighs .125 carats. Its mm measurements depend on the cut.