Gemstone Carat Weight

Carat weight and gem size are often mistaken for the same thing. To clarify the confusion and give you a better understanding of gemstone materials, we’ve put together a short article explaining the difference between the two.

Carat Weight Vs. Gem Size

Gem weight is measured on the metric carat scale. According to the scale, one carat is equivalent to 200 milligrams (0.20 grams), divided into 100 points for precision. For example, a sapphire weighing half a carat may be referred to as 0.50 carats or ‘5 points’. Thus, carat weight not only applies to diamonds but colored stones, too.

gem weight chart

There is a common misconception that carat weight represents visual size. However, they are not actually the same thing and should not be confused for two reasons, namely:

1. Two gemstones of the same proportions (i.e., shape and carat weight) may appear different due to their cuts. For example, gems with deeper cuts will likely appear smaller than those with shallower cuts.

2. Gems have different densities (mass per unit volume). Rubies and sapphires, for example, are denser than diamonds, meaning a one-carat ruby or sapphire will appear smaller than a one-carat diamond.

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

Gemstone Carat Weight Chart

Be aware that the carat weights and gem sizes in the following chart only apply to round diamonds cut to ideal proportions and that colored gems will have different weights than those listed below. This is because gem cutters will usually cut colored gems with a greater volume than diamonds when working on standard gem sizes.

Diamond Carat Charts

Should You Buy a Gem Based on Carat Weight or Size?

When buying gems, it’s important to consider what you need them for, as this will determine whether you need to buy based on millimeter (mm) size or weight. For instance, if you’re buying a stone for an existing jewelry setting, it’s best to buy based on mm size and not by estimating weights. On the other hand, if you’re buying a stone for a custom-made setting, you’re free to buy the stone that appeals most to you.

When buying gemstones from an online vendor, keep in mind their different densities. Don’t use diamond carat weights when buying colored stones or vice versa. Instead, order your gem based on mm size. This is because the varying densities of different gem materials and the depths of different cutting styles can result in a significantly different visual size in gems with similar carat weights.

Lastly, while it’s good to observe weight and size when buying diamonds and other stones, you shouldn’t limit yourself to these characteristics alone—also consider the gems as a whole and how they appear to the naked eye.

You can find a wide selection of loose diamonds and other gems at James Allen and Blue Nile.

Frequently Asked Questions

How big is a one-carat gemstone?

The standard size of a one-carat gemstone is 6.5 mm in diameter.

How do you determine carat weight?

The starting point for measuring the correct carat weight for a gemstone is to weigh it and take note of its weight in grams. Then, divide the number of grams by 0.2 to get the carat weight.

Is carat a measurement for weight or size?

A carat (ct) is a unit of weight used to measure diamonds and other gems. Most people tend to confuse carat weight (mass) with visual size (diameter or width).

What is a good carat size for an engagement ring?

Carat weights for engagement ring stones vary depending on the country you reside in. For example, in the United States, a diamond engagement ring can be between 1.08 carats and 1.2 carats. In Australia, on the other hand, it tends to be around one carat. Also, note that brilliant round-cut diamonds are the most popular choice for engagement rings in most Western countries.

However, there is no “ideal” carat weight or size for engagement ring stones, and the one you choose will ultimately depend on your personal preference and budget. Also, remember that carat weight influences price more than the color, cut, or clarity of the gem. So, if you’re looking to save money without compromising on appearance, opt for a lower carat weight.

Which gemstone is the heaviest?

The world’s heaviest gemstone is the Cullinan Diamond. It is the largest gem-quality rough diamond ever found, weighing 3,106.75 ct (621.35 grams 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, who was the chairman of the mine.

Which gemstone is the lightest?

Amber is said to be the lightest of all gemstones on earth—it is so light that it floats on salt water. It is formed from the resin of fossilized trees.

How do you weigh gemstones?

To calculate how much gemstones weigh, you need to use the metric carat scale. According to the scale, one carat (ct) equals 200 milligrams (or 0.2 grams), and gemstones are measured to the nearest hundredth for accuracy.

How many carats is a 6.5 mm diamond?

A round diamond measuring 6.5 mm in diameter is equal to one carat.

Which gemstone is the most expensive per carat?

The Blue Diamond is the most expensive of all the gemstones, priced at $3.93 million per carat.

How much is a 1.00 ct diamond?

Generally, a one-carat diamond is priced between $1,800-$12,000. However, diamond prices vary depending on the 4Cs (ct weight, color, clarity, and cut), with cut quality being the most important factor used in diamond appraisal.

How much is a one-carat sapphire?

A sapphire is not as expensive as a diamond, costing as little as $25 per carat to over $11,000 per carat. Depending on the quality, a one-carat blue sapphire can cost between $450 and $1,600.

How are gemstones measured?

There are three measurements for loose gemstones. These include diameter (for round gemstones) or length (for other shapes), carat weight, and density:

  • Diameter or length is measured in millimeters (mm) across the top of a gemstone as viewed from above.
  • Carat weight is measured in carats (ct), where one carat (1.00 ct) is equal to 200 milligrams or 1/5th of a gram. When looking at gemstones, it is important to remember that gem sizes do not necessarily reflect their weight and vice versa. Traditionally, gems are priced per carat.
  • Gemstone density (also called specific gravity) describes a gemstone’s weight relative to how it behaves in a liquid (i.e., how much it weighs in water). Different gem varieties have different densities, which can also influence their carat weight. For example, sapphire and ruby have a specific gravity of 4.0, while other gems like amethyst and emerald are lighter. A diamond has a specific gravity of 3.52, which means a one-carat diamond will not be the same size as a one-carat sapphire or emerald.

Thus, to get two varieties of gemstones that look the same, you need to disregard the gems’ carat weights and measure them according to their size in mm.

How much do gemstones weigh?

A gemstone’s weight is measured in carats, where one carat is equivalent to 200 milligrams. However, subtle variations in a stone’s density mean that no two varieties will ever weigh the same, even though they are the same size. In other words, a sapphire will always be heavier than a diamond of the same size.