The density of a gemstone will affect its carat weight.
The density of a gemstone, or its specific gravity, is an important concept to comprehend in order to understand why two different gemstones, of the same cut and physical size, can have two very different carat weights (a carat is a measurement of weight and not size).
Take four of the most precious gemstones in the world: Emerald, Sapphire, Ruby and Diamond. Cut them into identical shapes, and the Emerald will have the smallest carat weight, the Diamond will sit in the middle and the Sapphire and Ruby will be the heaviest. It is exactly the same principle as a 10cm cube of lead would weigh far more than a 10cm cube of Balsa Wood.
Every gem has its own unique construction and therefore has its own density. This measurement is known as the gem’s Specific Gravity (or SG for short) and is often used to correctly identify gemstones that are similar in visual appearance.
The chart on the next page should prove useful in helping you understand if you have two gems similar in size, which one will have the bigger carat weight. For example, a well-proportioned, round brilliant cut, one carat Diamond would normally be approximately 6.5mm in width, whereas a 6.5mm Emerald will weigh less than one carat and a Sapphire of the same size will weigh approximately 1.2 carats.
