Why are sapphires cut so deep?

[SouthernGent]

I've noticed that a lot of sapphires are relatively deep in their cuts compared to some other gemstones like diamond. Is there a particular reason for that?

 

[Avondale]

1. The material is scarce and maximising carat weight remains important in order to increase profits.

2. A deeper stone allows for light to have a longer travel path inside the stone which enhances the material’s colour. With diamonds the goal is exactly the opposite - to mask the colour as much as possible through optimised light return so that the stone faces up white.

 

[LilAlex]

^ These are the answers I know. I'll venture that the first that @Avondale lists is far and away the driver -- since sapphires are often overdeep even when it detracts from performance. And the columnar shape of the material often lends itself to weight retention in this orientation. That said, the color can be zoned to the exterior of the column (like wallpaper wrapped around an architectural column), in which case sapphires can be cut in the "wrong" orientation (not along the c-axis). I guess in that case, it's weight retention in the form of a deep, relatively colorless pavilion that otherwise could not be used for much.

 

[SouthernGent]

Thanks for the explanations, y'all.

 

[LunaIzzy]

I deal with hundreds of sapphires every day.

What I mostly see are sapphires cut for weight by rough dealers, primarily from Sri Lanka and Tanzania.

In Sri Lanka, they call it a mixed step cut, also known as a native cut. This cut tends to be deep, which makes the stone appear darker. By doing this, they can sell it at higher prices to cut and polished dealers.

The process flows as:
Miners > Rough Dealers > Cut and Polished Dealers > Wholesale > Retail..bla bla bla

 

[kenny]

Sapphires look deep to folks who are comparing them to diamonds, which are cut more shallow.

IR may explain why.
Index of refraction is how much light changes direction when it passes into or out of a gem.
The higher the IR the greater the change of direction.

Diamond's IR is 2.4.
But sapphire (corundum) is only 1.8.

Diamond's higher IR means the final gem doesn't have to be as deep as does a sapphire, at least when it comes to preventing windowing when viewing through the table.

 

[Starstruck8]

Sapphires look deep to folks who are comparing them to diamonds, which are cut more shallow.

IR may explain why.
Index of refraction is how much light changes direction when it passes into or out of a gem.
The higher the IR the greater the change of direction.

Diamond's IR is 2.4.
But sapphire (corundum) is only 1.8.

Diamond's higher IR means the final gem doesn't have to be as deep as does a sapphire, at least when it comes to preventing windowing when viewing through the table.

There must be more to it than that. Corundum RI is about 1.77, giving critical angle of about 34.4°. This is below typical diamond pavilion angles of about 40°- 43°. So these angles are steep enough to avoid a window in corundum, at least in square-on viewing through the table.

No doubt there are other issues. @Avondale's answer above seems plausible.

 

[Karl_K]

Look at the stone in the pic above its not a RB cut that can be compared to RB cuts in diamond.
Its one of the 50000 variants of the portuguese cut.
The same is true of the vast majority of them.

Why is the portuguese cut used?
1: retains weight.
2: can increase apparent color saturation
3: more facets = more reflectors helps lower RI stones sparkle more.
4: The bowl like pavilion helps some with tilt leakage of lower than diamond RI materials.
5: its a traditional design for sapphire/ruby now.

Diamonds have been cut in portuguese cut variants but they make no sense unless they are 50+cts.

So is that stone cut to deep? Depends, if its pretty and holds up under some tilt and has life its just right.

 

[Karl_K]

A window under the table in many portuguese cuts is from the bottom of the bowl like pavilion being too flat.
The larger the too shallow area the bigger the window.

 

[Karl_K]

Reading more; weight retention, color enhancement and RI differences are all part of the design differences for different materials.

 

[SouthernGent]

Thanks again for the professional explanations, y'all.

 

[PrecisionGem]

These overly deep sapphires are the ones we look for as good candidates for a recut. Typically, even though they are so deep, they still window as the last tier or two are cut to shallow. Perfect for a recut however.

Overly deep stones can present issues when setting too, and you end up with a stone sitting pretty high.

 

[SouthernGent]

These overly deep sapphires are the ones we look for as good candidates for a recut. Typically, even though they are so deep, they still window as the last tier or two are cut to shallow. Perfect for a recut however.

Overly deep stones can present issues when setting too, and you end up with a stone sitting pretty high.

And that is exactly why I brought up the question, as I'm looking for a durable blue gemstone for a men's ring and whenever I come across a sapphire that I like it is always so deep that it would not likely fit in a ring without it sitting way up high.
I may look at a blue diamond instead.

 

[RRfromR]

These overly deep sapphires are the ones we look for as good candidates for a recut. Typically, even though they are so deep, they still window as the last tier or two are cut to shallow. Perfect for a recut however.

Overly deep stones can present issues when setting too, and you end up with a stone sitting pretty high.

@PrecisionGem hmmm, maybe this is what I should do with mine before setting it.

 

[Karl_K]

@PrecisionGem hmmm, maybe this is what I should do with mine before setting it.

I'm not seeing a window in the center so it is all ready well cut.

 

[Dreamer_D]

I'm not seeing a window in the center so it is all ready well cut.

Yeah it looks great and if it looks great in person don’t mess with it!

 

[Gloria27]

I've noticed that a lot of sapphires are relatively deep in their cuts compared to some other gemstones like diamond. Is there a particular reason for that?

Because these are native cuts and are meant to save weight and preserve the value of the stone but also for cultural reasons, the bigger the stone the more powerful the astrological effects etc.