(Closed) Can someone help me understand carat weight and mm?

posted 6 years ago in Rings
Post # 3
9641 posts
Buzzing Beekeeper
  • Wedding: November 2010

I think only diamonds are measured in carats.  If you go to JewelryTV’s website, JTV.com, they have a TON of educational ariticles.  I can’t copy and paste from this computer or I would find you a link.

But the site is easy to navigate.  you may want to check out Jewel School.

JTV is my latest obsession.  Lucky for me, Darling Husband likes gem stones, so he doesn’t mind.








Post # 4
3689 posts
Sugar bee
  • Wedding: December 2011

Carat weights don’t carry across stone types because they don’t have the same densities.  Measuring the size in mm, though, is universal.

Post # 5
4352 posts
Honey bee
  • Wedding: August 2013

@strawbabies:  +1. Carats measure the mass of a stone, which is based on its density and densities vary across stone types. I would only use carats as a guide if you’re comparing two of the same type of stone (diamond and diamond, sapphire and sapphire ect.). You can compare how large diamond will look compared to a sapphire with the mm. One thing you will hear people say is that their stone “faces up small” or “faces up large” which means that it has a wider/smaller table than most stones of that size. For example, my asscher is 1.06ct, and is 5.4mmx5.4mm. Most 1ct round diamonds are 6.5mm. Which means if I compare mine to someone else’s round, mine will look smaller from the top (and larger from the side). Most fancy cuts (asscher, cushion, princess, radiant) usally face up small and some face up large (marquise).

Post # 7
585 posts
Busy bee

All stones are measured in carats because carat is a measure of mass. It’s a very small measure. Gems are a little different from each other in terms of density so two different gems of the exact same physical size and shape may not weigh the same, just like a brick and a piece of foam the same size won’t weigh the same.

The mm is for millimeters which is a measurement of length. 10 mm = 1 cm.

You are correct that your 6×8 aquamarine and a 6×8 oval moissanite will look the same size from the top (sizewise), but the moissanite is heavier, so the carat weight is more. I think both the 1.4 ct (6.5×6.5) and 1.8 ct (7×7) will give you a very similar finger coverage to what you have now, so just decide if you want to err on the side of bigger or smaller. 🙂 Show us pics of what you decide!!

Post # 8
4475 posts
Honey bee
  • Wedding: November 2012

@frogprincess:  Can’t put it better than this!


The one thing I’ll add is moissanite carat weight can be a tad confusing.  On moissanite websites, the carat weight listed is not usually the true moissanite carat weight, but the diamond equivalent.  In other words, the carat weight online is usually the carat weight a diamond that size would be.  A 6.5mm round brilliant moissanite is listed as 1 carat, because an ideal 6.5mm round brilliant diamond is 1 carat.  In reality the carat weight’s something else.  If you buy that stone and get your paperwork, you might have a different number listed as the carat weight.  That’s ok, in this case you’ll go by the stone dimensions.

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