The origin of the word carat comes from the carob seed. Gemstones were once weighed on scales and dealers used carob seeds as counter weights (the carob seed has uniform shape and mass). In the early 20th century carat weight was standardized so .2 of a gram
1 carat = 200 milligrams or .200gm
1 carat = 100 points – so a point is 1/100 of a carat
Jewellers sometimes refer to a diamond in points. A 25 point (25pt) diamond is a .25ct or 1/4ct
Carat weight does not equal size because the visible size of the diamond can be affected by the cut. A diamond that is cut very deep may weigh more, but appear smaller, than a diamond that is cut shallow and wide. Excess depth is undesirable in round stones because the stone reflects less light (see the diamond cut section below for more details).
In terms of price, all 4 C’s affect the price of the stone. The price is always impacted differentially by each C, especially cut and carat.
Diamond prices go up in steps at certain weights. Prices jump at .50 carat, .75 carat, 1.00 carat and so on. 1:00 carat stones are very popular and diamond cutters try and get their stones to reach this magic number.
One other important thing to remember is that diamond weights are rounded up from the third decimal point only when it is a nine.
0.995ct = 0.99ct
0.998ct = 0.99ct
0.999ct = 1.00ct
This means there can be .005 of a carat difference in the reported carat and actual carat.
Other important things to note about carat weight
In a ring that has been pre-set with a diamond you cannot accurately measure the weight (unless you take the diamond out of the ring). Therefore most times the weight is estimated taking the dimensions of the stone, with a margin of error of around 5%. It is also important to note that many retailers advertise the total weight of the diamonds on the ring. This may include the lower quality smaller diamonds that are sometimes used to decorate the ring setting. Beware of these techniques and if you are buying a ring setting with decorative diamonds make sure that they are of similar quality to the main diamond stone in your ring.