Precious gemstones vary in density, for example when comparing a diamond and a sapphire of the same size, the sapphire will be heavier due to it's larger density. Gemstones are often classified by carats, which is a unit of weight, but for ease of selection and setting, size is a more useful measurement for sapphires, which we list in millimeters (mm). Our standard size for a round sapphire is 6 mm, which is approximately one carat. Download the chart below and print it out for your own reference when selecting the size.