Carat (ct)

Weight, not size

Carat measures mass, not diameter. Two stones of the same carat weight can face up very differently, because cut proportions decide how much of the weight sits in the visible top of the stone and how much hides in the pavilion below. A well cut round brilliant of 1 ct measures about 6.4 to 6.5 mm across, while a deep stone of the same weight looks smaller from above.

Different gem materials also weigh differently at the same size. Sapphire and ruby are denser than diamond, so a 6.5 mm ruby weighs more than a 6.5 mm diamond. Moissanite is lighter, so it weighs less at the same diameter.

Carat in CAD and renders

CAD files do not store carat weight. They store geometry, so a stone in your model is defined by its diameter, length, width and depth. When you prepare a model for rendering, size the stones to the millimetre dimensions of the real stones you plan to set. Our diamond size chart converts carat weight to millimetres for round brilliants.

Accurate stone sizes matter in product images. Customers compare the centre stone with the band width and the prongs, and a stone that is too large or too small in the render looks wrong next to the finished piece.

Carat versus karat

Carat with a C is gemstone weight. Karat with a K describes gold purity in the United States, although many countries, including the UK and India, also write gold purity as carat, for example 18ct.