Why Don't Moissanites and Diamonds with the same Millimeter Size Weigh the Same?

Most minerals have different atomic weights, so if you go by weight when comparing stones, a one-caret round brilliant gemstone, say an Amethyst, would not necessarily be the exact same size as a round brilliant one-caret Moissanite or Garnet. To make things easier on the public, long ago the jewelry industry established certain "ideal sizes" for diamonds and gemstones in all loose stone shapes and sizes such as pear, heart, round brilliant and so on. This way the jewelry manufacturers could produce standard findings (jewelry parts) and the loose stone stone cutters would know what sizes would best fit those findings. The closer a stone comes to the "ideal" cut for that size and shape, the more the stone is worth in general.

So if you go strictly by weight, a one-caret Moissanite would not be the same size as a one-caret Diamond. BUT, if you go by the standard dimensions, in this case millimeters, a 7-millimeter round brilliant cut Moissanite loose stone would be exactly the same SIZE as a 7-millimeter round brilliant cut loose stone Diamond. Because most people are familiar with the shapes and caret weights of standard size Diamonds, and most people who buy loose stone Moissanites want them to look like diamonds, Charles and Colvard cuts their stones to be the equivalent SIZE of standard size Diamonds, even though the weight will be different. Here at Mrs. Gottrocks, we carried that one step further in order to lessen the confusion for our customers, so we quote the WEIGHT of the stone in Diamond weight as well. But as you can see from the Millimeter Sizes and Weights for Loose Stone Round Brilliant Cut Moissanites and Equivalent-Sized Loose Stone Diamonds table below, the actual weight in carets of the Moissanite would be less if you weighed it, but the dimensions of the loose stone are the same as for the diamond loose stone.