Gem 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 article to explain the difference between the two.


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.

gem weight

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.


The carat weights given for standard gem sizes in the chart apply to diamonds cut to ideal proportions. Usually, gem cutters will cut colored gemstones with greater volume than diamonds so they may weigh more than as described below.

Diamond Carat Chart

Frequently Asked Questions

Although sizes will vary slightly depending on the cut, a 1.00 ct diamond or gemstone in a round brilliant cut will be approximately 6.5 millimeter in diameter. However, this is not a scientific figure, and there can be slight variations.
The first step that you should take is to take measurements the diameter of a round diamond and then using a size chart; you will need to find the value which is closest to your measurements 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 measurement with one-carat equalling 0.2 grams or 200 milligrams. Generally, a one-carat round brilliant cut diamond would be 6.5 millimeter in diameter although the size can vary whereas the weight cannot.
There are no 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.
The largest precious gemstone ever found, is the unfaceted Bahia Emerald in Brazil. The gemstone weight is approximately 840lbs (381kg) and contains more than 180,000 carats.
Amber is the lightest and softest gem.
Grams to carats: One carat = .2 grams or 200 milligrams, equal to 1/5 of a gram. Stones are measured to the nearest hundredth of a carat. A hundredth of a carat is also called a point. Thus a .10 carat stone can be called either 10 points, or 1/10 of a carat. You can find a gemstone carat weight calculator online to determine stone weight measurement.
The diamond carat size will be about 1 carat.
These five gemstones are the most expensive per carat:
1. Blue Diamond – $3.93 million per ct
2. Jadeite – $3 million per ct
3. Pink Diamond – $1.19 million per ct
4. Red Diamond – $1,000,000 per ct
5. Emerald – $305,000 per ct
The price of a 1 ct diamond is between $1,300 – 16,500, depending on the diamond’s cut quality, clarity, color and shape.
A blue sapphire around 1 carat is likely to cost from as little as $450 to $1,600, depending on the quality of the gem.