Gem Weight

Carat weight and diamond gem sizes are often thought to be the same thing. To clear the confusion and give you a better perspective on gemstone materials, we’ve put together a small article to explain the difference between these two.


Gem weight is measured in metric carat scale. One carat is equivalent to 200 milligrams (0.20 grams) and is divided into 100 points for precision. This applies not only to diamonds but to colored stones, too. A sapphire weighing half of a carat may be referred to as 0.50 carats.

gem weight chart

Carat weight and size are not the same and these two 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 gems with deeper cuts are likely to appear smaller than the other.

2. Gem have different densities (mass per unit volume). Sapphires, for instance, are denser than diamonds, meaning one-carat sapphires would appear smaller than one-carat diamonds.

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, when working on standard gem sizes, gem cutters will cut colored gems with greater volume than diamonds so there will be different weights as those listed below.

Diamond Carat Charts


When buying gems it’s important to know what you need them for, as this will help you decide on the determining factors. If you’re buying a stone for an existing jewelry setting it’s best to buy by size and not by estimating weights. If you are buying a stone to have a setting custom-made then you buy the stone that most appeals to you.

It’s good to note weight and size when buying diamonds or other stones but don’t limit yourself to these characteristics alone. Consider the gems as a whole, even the way it looks to the naked eye.

Frequently Asked Questions

1-carat gemstones in standard sizes are 6.5 mm in measurement.
The starting point in determining a correct carat weight measurements of gemstones is to weigh the gem and note down its weights in grams. Divide the gram number it weighs by 0.2 to get the carat weight of the diamond gem.
A carat (ct) is one of the weight measurements, that is, one fifth (1/5) of one gram, that is used on diamonds and other gems. It is a common for most people to confuse the carat weight with the visual size. The weight will visually vary depending on the shape and quality type of gemstone.
The US average for a diamond engagement ring is between 1.08 and 1.2 carats, while the Australian popular request is 1-carat, a popular consideration of a classic diamond engagement ring. Note that a brilliant round diamond is the popular choice.
However, it is important to remember that a diamond’s beauty is not affected by mere carat weight alone. There is also clarity, and cut and color. There is also a tip of going for a size (or its diameter or width) just below the larger stone you wish for. The cost of diamonds will vary for each 0.5 ct.
The world’s heaviest of gemstones is the Cullinan Diamond. It is described as the largest gem-quality rough diamond ever found, it weighs 3,106.75 ct or 621.35 g or 21.9 ounces. Discovered in January 1905 at the Premier No. 2 mine in Cullinan South Africa, the stone was named after Thomas Cullinan, chairman of the mine.
The softest and lightest among gemstones on earth is said to be Amber – it is so light it floats on salt water. Amber is formed from the resin and sap material combination in fossilized prehistoric trees.
To calculate how much the gemstones weigh, according to the metric carat scale, one carat ct is equal to .2 grams or 200 milligrams, equal to 1/5 of one gram. Gemstones are measured in weight to the nearest hundredth carat.
A diamond measuring 6.5 mm round is equal to 1.03 carat weight.
The Blue Diamond is the most expensive among all gemstones at $3.93 million per carat.
Generally, a 1-carat diamond is priced between $1,800-$12,000. Diamond prices vary depending on each of the 4 Cs and the cut quality is the most critical factor for a 1 ct diamond’s beauty, and consequently, price.
A sapphire is not as expensive as a diamond, costing as low as $25 – $11,000 per carat. A 1-carat blue sapphire price can go from $450 to $1,600, varying cost by quality.
To weigh a gemstone is by carat, 1 carat is equal to .20 grams or 200 milligrams. Gemstones less than 1 carat are expressed in units 100 times smaller, referred to as points, like 50 points is equal to 0.5 carats.
Since gemstone size is expressed in weight, gemstones are measured in metric carat, equal to 1/5 of one gram. Traditional gem prices were per carat price.
At the same time, the gems specific gravity (that is, how much it weighs in water) is not the same. Specific gravity of a diamond is 3.52. Other gems like amethyst are lighter weight with the specific gravity of 2.66. Sapphire is 4.0. All gemstones weigh differently. A 1 carat diamond will not look the same size as a 1-carat amethyst or sapphire.
So the correct measure of the gems’ worth is by millimeter size, like 4×6 mm or 5×7 mm. Thereby a 4×6 mm sapphire will look actually the same size as a 4×6 mm amethyst, regardless of the gems’ weights.
One carat is equal to 0.200 grams or 1/5 gram, subdivided into 100 points (for less than 1-carat gems).