Specific size questions about colored gemstones/shapes...

[matt_k]

Hi, gang. I'm sorry if this is a redundant thread, but I was hoping you could help me with translating a colored gemstone's millimeter size to what it translates to visually in terms of diamond sizes. Like, I already know what 1, 1.5, and 2 carat round brilliant diamonds look like like in terms of face-up size and finger coverage. but could you help me translate the equivalent face-up size in millimeters for gemstones?

From what I've been able to figure out online, I think the following measurements are correct as far as what a colored gemstone looks like in comparison to a round brilliant diamond, but if anybody has any corrections to make I would love to hear 'em.


For oval shaped gemstones ( in particular oval cabachons)

7×5mm: visually equal to 1 carat RBC
8×6mm: visually equal to 2 carat RBC
9×7 mm: visually equal to 3 carat RBC
10×8mm: visually equal to a 4 carat RBC

Providing those comparisons are roughly true, could somebody help me figure out the formula for cushion-cut cabochon gemstones in comparison to the visual face up size (and finger coverage) of RBCs?

 

[qubitasaurus]

So different types of gemstones have different densities. For instance emerald is a lot less dense than sapphire, and so a 1 ct emerald faces up much larger than a 1 ct sapphire. Also there are at least two different types of cushion shaped cabochon (sugarloaf and ones with a flatter dome), and they'll both face up differently too for the exact same ct. weight. Actually in general you should be looking at height of the dome when selecting a cab -- dont buy a pancake shaped stone with a very flat dome.

So your not going to be able write down a simple one size fits all formula.

But gemstones are more sold on mm size anyway, so maybe you can go off this

Cushion Cut Diamond Size Chart (Carat Weight to MM Size)

Is your Cushion cut diamond too deep? Use our diamond size chart guide to determine the appropriate millimeter size (dimensions) of a cushion diamond based on its carat weight.

www.brilliance.com

And just match the dimensions?

 

[matt_k]

So different types of gemstones have different densities. For instance emerald is a lot less dense than sapphire, and so a 1 ct emerald faces up much larger than a 1 ct sapphire. Also there are at least two different types of cushion shaped cabochon (sugarloaf and ones with a flatter dome), and they'll both face up differently too for the exact same ct. weight. Actually in general you should be looking at height of the dome when selecting a cab -- dont buy a pancake shaped stone with a very flat dome.

So your not going to be able write down a simple one size fits all formula.

But gemstones are more sold on mm size anyway, so maybe you can go off this

Cushion Cut Diamond Size Chart (Carat Weight to MM Size)

Is your Cushion cut diamond too deep? Use our diamond size chart guide to determine the appropriate millimeter size (dimensions) of a cushion diamond based on its carat weight.

www.brilliance.com

And just match the dimensions?

Thanks!

 

[arkieb1]

Add to that coloured stone ovals, native cut, precision cut and cabs vary a lot in depth so cut stones can be shallow or bottom heavy and cabs the reverse, some are flatter or they can be really high domed which means for each carat size there isn't one accurate mm set of dimensions.

There is a standard range of popular sized mass produced settings, like 10 x 8 for example and often mass produced stones cut in places like China or India are cut size wise specifically to go into those settings, so it's not impossible to find stones that are already specific mm sizes it just depends on the types of stones ie how rare they are and who is cutting them, so if you want something common like an Amethyst or a citrine or a topaz it's easy to find them in specific mm sizes, but, if you want something rarer like some types of spinel or say a Burmese sapphire a specific shade of blue..... not so much.

 

[Bron357]

Carat is a weight and is determined by both size and density of the gemstone. Different types of Gemstones have different densities so there is no “one rule suits all”.
Measurements are easier to compare, so look at the gems dimensions to determine whether the size is going to say fit a setting or provide sufficient finger coverage.
That said, gems are usually sold as a “per carat” price and some cuts give a bigger “look”.
For eg an OEC (old European cut) diamond has a big high table (the top section) so for its “carat” it looks smaller than a round brilliant. Marquise cut diamonds probably provide the biggest “visual size” for their carat weight.
With Coloured gems they are usually cut for colour and weight, not light refraction as a diamond is. So they might have a thick girdle (gemstone edge) and a big bottom underneath. This is carat weight you’ll be paying for but won’t be visible to appreciate. So always check the depth measurement as well with coloured gems.

 

[matt_k]

Carat is a weight and is determined by both size and density of the gemstone. Different types of Gemstones have different densities so there is no “one rule suits all”.
Measurements are easier to compare, so look at the gems dimensions to determine whether the size is going to say fit a setting or provide sufficient finger coverage.
That said, gems are usually sold as a “per carat” price and some cuts give a bigger “look”.
For eg an OEC (old European cut) diamond has a big high table (the top section) so for its “carat” it looks smaller than a round brilliant. Marquise cut diamonds probably provide the biggest “visual size” for their carat weight.
With Coloured gems they are usually cut for colour and weight, not light refraction as a diamond is. So they might have a thick girdle (gemstone edge) and a big bottom underneath. This is carat weight you’ll be paying for but won’t be visible to appreciate. So always check the depth measurement as well with coloured gems.

Thanks so much for the info! On average, would you say a cabochon cut (oval, round, or cushion) packs a pretty good visual wallop as far as finger coverage goes? From what I've been able to research online, it looks like a 6 mm oval cabochon faces up nice and big.

 

[matt_k]

Add to that coloured stone ovals, native cut, precision cut and cabs vary a lot in depth so cut stones can be shallow or bottom heavy and cabs the reverse, some are flatter or they can be really high domed which means for each carat size there isn't one accurate mm set of dimensions.

There is a standard range of popular sized mass produced settings, like 10 x 8 for example and often mass produced stones cut in places like China or India are cut size wise specifically to go into those settings, so it's not impossible to find stones that are already specific mm sizes it just depends on the types of stones ie how rare they are and who is cutting them, so if you want something common like an Amethyst or a citrine or a topaz it's easy to find them in specific mm sizes, but, if you want something rarer like some types of spinel or say a Burmese sapphire a specific shade of blue..... not so much.

Thanks!

 

[woweezowee]

Matt, it depends on the type of setting and finger size. I find personally that cabochons (aka cabs) appear smaller than their faceted counterparts of the same dimensions. Is the oval 6 mm on the smaller dimension?

Also, if set in a bezel, which I think is always an A+ way to set a cab, that'll make the finger coverage larger due to the metal encompassing the stone, as opposed to a prong setting.

My ring finger is a size 6.75 and a bezel-set round cab would be enough finger coverage for me. If I put it on my 8.5-size middle finger, not as much.

FWIW, I LOVE cabs and wish there was more fine jewelry made with them. For example, I'd take a cab cut neon pink spinel over a faceted specimen any day.

 

[matt_k]

Matt, it depends on the type of setting and finger size. I find personally that cabochons (aka cabs) appear smaller than their faceted counterparts of the same dimensions. Is the oval 6 mm on the smaller dimension?

Also, if set in a bezel, which I think is always an A+ way to set a cab, that'll make the finger coverage larger due to the metal encompassing the stone, as opposed to a prong setting.

My ring finger is a size 6.75 and a bezel-set round cab would be enough finger coverage for me. If I put it on my 8.5-size middle finger, not as much.

FWIW, I LOVE cabs and wish there was more fine jewelry made with them. For example, I'd take a cab cut neon pink spinel over a faceted specimen any day.

I agree! I'm actually working on a project using high quality tanzanite oval cut Cabochons( great color on these puppies, plus you can go down a bit on clarity due to the cut--- creating a much more affordable set of Tanzanite earrings than if I'd gone the faceted route).

 

[Niffler75]

@matt_k It's against the rules to tell us about a project and not share pics! I love cabs too. There are some fantastic examples of cabs used in fine jewellery designs!

 

[prs]

@matt_k you can easily calculate the face up area of any shaped stone. The following calculations will give you the face up area in square mm.

Round: 3.142xDxD/4
Oval: 3.142xLxW/4
Square: LxW
Rectangle: LxW

 

[arkieb1]

If it's 6 mms across ways then that is fine if it's 6mms up and down (so the length) generally about 7.50 to 8.00mms is where it's getting a decent size....

 

[Bron357]

I do so hope they are dangle earrings!
There is just something magical about a beautiful coloured cabochon when the light can also come through from behind.
If you have 4 cabochons (ie 2 x 2 of similar size and shape), I’d do a cabochon stud at the earlobe and dangle the other on a chain an inch or so off the stud.

 

[SapphireNut1]

There are online gem weight calculators that allow you to put in a bunch of variables- shape, girdle thickness, mineral type, and it will calc out the approximate weight. You need to use something like that. Colored stones have wildly different densities between mineral types. Feldspar is pretty light (Oregon sunstone, bytownite), sapphires are dense. When I'm cutting my design software will calc the weight of the stone.

Girdle thickness can add a lot of weight, and with some pale gems I'll use a design with a deeper pavilion to help with color saturation. For example a 5mm round brilliant sapphire will usually be about a half carat, but I have some round designs that can yield nearly a carat in a 5mm stone. You don't get face up size with that design but you'll get better color out of pale unheated Montana's

This is a handy calculator that's pretty accurate

Precise Gemstone Carat Weight Calculator from Dendritics

dendritics.com

 

[matt_k]

@matt_k you can easily calculate the face up area of any shaped stone. The following calculations will give you the face up area in square mm.

Round: 3.142xDxD/4
Oval: 3.142xLxW/4
Square: LxW
Rectangle: LxW

Math is NOT a strength of mine, sadly. I'm more of a goofy humanities guy.

 

[matt_k]

@matt_k It's against the rules to tell us about a project and not share pics! I love cabs too. There are some fantastic examples of cabs used in fine jewellery designs!

My project went down the toilet, unfortunately. Oof. The tanzanite cabs I received for the earrings project looked nothing like the pics ( instead of that nice deep blue-violet they were a watered down purple--- like weak, ghetto-ass amethysts).

 

[matt_k]

There are online gem weight calculators that allow you to put in a bunch of variables- shape, girdle thickness, mineral type, and it will calc out the approximate weight. You need to use something like that. Colored stones have wildly different densities between mineral types. Feldspar is pretty light (Oregon sunstone, bytownite), sapphires are dense. When I'm cutting my design software will calc the weight of the stone.

Girdle thickness can add a lot of weight, and with some pale gems I'll use a design with a deeper pavilion to help with color saturation. For example a 5mm round brilliant sapphire will usually be about a half carat, but I have some round designs that can yield nearly a carat in a 5mm stone. You don't get face up size with that design but you'll get better color out of pale unheated Montana's

This is a handy calculator that's pretty accurate

Precise Gemstone Carat Weight Calculator from Dendritics

dendritics.com

Thanks!!! Sounds like you really know your colored gemstones, good sir.

 

[SapphireNut1]

Thanks!!! Sounds like you really know your colored gemstones, good sir.

USFG certified grand master faceter. It's a hobby for me though not my livelihood

 

[Niffler75]

@matt_k Oh no,that's really disappointing about the cabs! Are you sending them back?

 

[LemonMoonLex]

"Ghetto-@s$ amethysts"

 

[matt_k]

USFG certified grand master faceter. It's a hobby for me though not my livelihood

right on, right on!

 

[matt_k]

@matt_k Oh no,that's really disappointing about the cabs! Are you sending them back?

yep.

 

[matt_k]

"Ghetto-@s$ amethysts"