Millimeters, by definition, have no carat weight.
Your answer depends on the cut you are measuring. If the gem is a round cut, the carat weight will be about .10 carats.
A 3mm diamond may imply its dimension at the girdle. If the girdle is on a round diamond, this diamond may measure in the range of 1/10th of a carat.
3x1250x5000
Carat is the measurement of a diamond's weight. The stone's value would partially depend on its carat weight.
Carat is the weight measurement for gemstones.
A carat is a measurement of weight. The biggest carat, then, would be the weight of the biggest item being weighed using carats.
Carat is a weight, so its weight is one tenth of a carat or one tenth of .20 grams.
A carat is a unit of weight used for gemstones. One third carat is the weight of the diamond.
You can calculate the total carat weight by adding the individual carat weight of each stone. Without a carat weight of each of your 15 stones, it is not possible to devise a sum total carat weight.
One carat of diamond in carat total weight equals one carat. One thousand dollars worth of diamond in total carat weight depends on the cut, colour, clarity and individual carat weight of the stones in the group.
If it is 1 carat weight=.20g .5ct= .10gms and so on you need to know how much it is carat the multiply carat with .20g
Carat is not a measurement of size, its weight. 200 miligrams=1 carat