Gemstone Carat Weight

Carat weight and diamond size are very often thought to be the same thing. To clear any confusion, we’ve put together a small piece that explains the difference between the two.


Many Colorful Gems

The weight of a gemstone is measured in carats. One carat is equivalent to 200 milligrams (0.20 grams) and is divided into 100 points for precision. A sapphire weighing half of a carat may be referred to as 0.50 carats.

Carat weight and size are not the same and must not be confused for two reasons:
1. Two gemstones of the same type, carat weight, and shape may appear different due to differing cuts. The deeper cut stone is likely to appear smaller than the other.
2. Gemstones have differing densities (mass per unit volume). Sapphire, for instance, is denser than a diamond, meaning a one-carat sapphire would appear smaller than a one-carat diamond.

The best way to visualize the size of a gemstone is by its dimensions, the top diameter in particular.


Although sizes will vary slightly depending on the cut, a one-carat gemstone in a round brilliant cut will be approximately 6.5mm in diameter. However, this is not a scientific figure, and there can be slight variations.
The first step that you should take is to measure the diameter of a round diamond and then using a sizing table; you will need to find the value which is closest to your measurement in millimetres. For instance, a 4.1mm diamond would be 0.25 carat, and a 6.5mm diamond would be one-carat.
A carat is a unit of weight with one-carat equalling 0.2 grams or 200 milligrams. Generally, a one-carat round brilliant cut diamond would be 6.5mm in diameter although the size can vary whereas the weight cannot.
There are no hard and fast rules regarding how many carats an engagement ring should be. However, the average size of an engagement ring diamond is between half a carat and one-carat. The trend, just like the expected spend, has increased over the years and s continuing to do so.