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Gemstone Measurements and Millimeter Versus Carats

Gemstones are an interesting topic when it comes to measurements. Sometimes you’ll see gemstones listed as millimeter (MM) size (often misspelled as milimeter), like 6 x 4mm, and other times you’ll see gemstones listed as a carat weight, like 1.00 carats.

What Gives?

What makes gemstones, like sapphire, ruby & emerald, so different that they’re measured differently? Let’s find out…


Gemstones have many unique attributes that make them so different from diamonds. When it comes to diamonds, most people know and understand how diamonds are weighed. They’re used to carats and carat sizes and can easily relate to them.

But, when it comes to gemstones they have no clue. The biggest reason why gemstones are measured differently than diamonds are quite simple:

They don’t weight the same.

More accurately, their specific gravity is different. Diamond’s specific gravity (how much it weighs in water) is 3.52. Some gemstones like amethyst for example are lighter weight and have a specific gravity of 2.66. Sapphire on the other hand is 4.0. All gemstones weigh differently.

So a one carat (1.00) diamond will not look the same size as a one carat amethyst or a one carat sapphire. So if all gemstones were weighed in carat sizes, they would all look like different sizes.

It would get very confusing, very fast.

People may think that they’re getting a big gemstone, only to find out it looks small.

Carat weights in gemstones just don’t work well. Jewelers only sell them that way because people are accustomed to shopping for carat weights. It just makes sense.

Millimeter sizes:

The correct way to buy gemstones is by millimeter size like 4 x 6mm, or 5 x 7mm. That way, a 4 x 6mm sapphire will actually look the same size as a 4 x 6mm amethyst, no matter what the weights are.

Plus, most mountings sold for gemstones are set up for millimeter sizes, not carat weights. Just look in any jeweler’s catalog and you’ll see. A mounting like a mother’s ring for example may call for gemstones that measure 2.5mm each.


Not only are gemstones weighed differently than diamonds, but gemstones are cut differently as well. Most gemstones are much deeper than a diamond is. In fact, gemstones need to be cut deeper to help bring out their vivid color. If a gemstone was cut like a diamond, chances are it would appear light in color and even look transparent. They’ll lack those intense dramatic hues that gemstones are known for.

So when it comes to shopping for gemstones, forget about carat sizes, that’ll do you no good. Look at the millimeter sizes and the intensity of the color instead. In most gemstones, the darker the color, the better.

Here’s a chart that compares the millimeter sizes of diamond and sapphire to their corresponding carat weights.

Compare weights:

Millimeter Sizes of Diamond Versus Sapphire Gemstones

Now lets look at standard gemstone millimeter sizes for oval stones versus their carat weights (sizes will vary slightly depending on the gemstone’s specific gravity).

Oval gemstones:

  • 5 x 3 mm = .25 ct
  • 6 x 4 mm = .50 ct
  • 7 x 5 mm = 1.00 ct
  • 8 x 6 mm = 1.50 ct
  • 9 x 7 mm = 2.50 ct
  • 10 x 8 mm = 3.00 ct
  • 11 x 9 mm = 4.00 ct
  • 12 x 10 mm = 5.00 ct
  • 14 x 10 mm = 6.00 ct
  • 14 x 12 mm = 8.00 ct
  • 16 x 12 mm = 10.5 ct
  • 18 x 13 mm = 14.2 ct
  • 20 x 15 mm = 20.3 ct
  • 22 x 15 mm = 23.5 ct
  • 23 x 17 mm = 32.2 ct
  • 25 x 18 mm = 43 ct
  • 27 x 20 mm = 58.5 ct
  • 30 x 22 mm = 79.6 ct

As you can see, gemstone millimeter sizes can get quite large. Now in comparison, let’s look at some standard carat weights for round diamonds and their millimeter sizes.

Carat weights:

  • 3.0 mm = .10 ct
  • 3.8 mm = .20 ct
  • 4.1 mm = .25 ct
  • 5.2 mm = .50 ct
  • 5.9 mm = .75 ct
  • 6.5 mm = 1.00 ct

Interesting, eh?

Check out these loose gemstones HERE on Amazon.

Cheers! :)

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