Gem Weight

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


Gem weight 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 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 differing densities (mass per unit volume). Sapphire, for instance, is denser than a diamond, 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, gem cutters will cut colored gems with greater volume than diamonds so their weights may not be the same 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 weight. 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 but don’t limit yourself to these characteristics alone. Consider the gem in total, even the way it looks to the naked eye.

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 millimeters 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 of 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 millimeters. 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, one-carat round brilliant cut diamonds 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.
Gems are weighted in carats the same as diamonds. However, gems have different densities. Some are denser than diamonds while others are lighter. This will impact the weight.
Gems are typically measured by millimeter size. While they also have a carat size most mountings for precious gems are set up for millimeter size and not carat weights. For example, sometimes you will see a ring mounting calling for a 2.5mm stone. That’s the size of the gem you need.
It depends on the gem. Gems all have different densities which will impact their weight. For example, a sapphire is denser than a diamond which means that the same carat weight sapphire may appear smaller than a diamond. Other gems that are lighter than diamonds, like amber, will appear bigger.