Does CROWN height matter?

[PrecisionGem]

Many people cutting stones pay little regard to the crown height or angles, as the pavilion does most of the work in a gemstone. However low crowns typically lower the performance of the stone.
Here are some ray trace plots of a pretty standard round design showing the effect of a lower crown.

The original design first, with the first tier crown angle cut at 44 degrees.



Now changing the crown by just 5 degrees to 39 degrees.



Notice the large area of extinction and less dispersion. (Dispersion is shown as the ged/green/orange area's)

Lowering the crown even further to 29 degrees, produces even more trouble!



Very often you see stones with very flat crown from commercial cutting houses. This is often done to maximize the face up size of the stone.

 

[2Neesers]

Thanks for sharing this @PrecisionGem ! I have recently started paying attention to crown height, but didn’t know if I actually needed to with CS. Can it cause performance issues if the crown is overly high?

 

[Heidi2101]

This is really informative. As a newbie to coloured stones, these pics really helped.

 

[mtsapphirelovingannie]

Thank you Gene for the information.

 

[corundum_conundrum]

Oddly #3 appears to have the most dispersion.

 

[Girl_in_green]

Thank you, Gene. That was very informative.

 

[BlingObsession]

Oh that was very useful - thank you.

 

[elle_71125]

Great post Gene. I’ll admit that crown height isn’t something I’ve often considered in CS but your pics really bring to light how important it is.

 

[prs]

@PrecisionGem thanks for the info. Your post tells us that for colored stones 44° is good for the crown angle, and that more shallow angles are not so good. I know that for diamonds a crown angle range of 32-36° is considered optimum in combination with a pavilion angle range of 40-41°. Are there similar optimum ranges we should be looking for in colored stones, and would that range vary depending upon the refractive index of the stone?

 

[PrecisionGem]

The correct angles will change depending on the gem material, and also overall design and pavillion. There is no set formula that can be universally applied. Angles that will work with this round design, may not work with a rectangular design.

The best judge of a stone is your eyes. Often people will ask about proportions or angles to try to determine if a stone will perform well, but it’s not that simple. There are no set ratios or angles that are universal, and always your best assessment is made by looking at the stone.

I use software to model every design and then tweak the angles depending on the material. For some materials with low refractive indexes, all the tweeting in the work won’t make a particular design work.

For material with Refrative Index (RI) below 1.6 you are really limited with what can be done and still have a lively performing stone. With higher refractive index materials such as diamonds, these angles actually become less critical as there is more flexibility to produce a well performing stone.

 

[prs]

all the tweeting in the work won’t make a particular design work.

@PrecisionGem yes, we all know that too much tweeting can be a problem!

 

[MissyBeaucoup]

This was very helpful, Gene. Thank you. Is it conversely true that a higher crown with a smaller table can add sparkle?

 

[PrecisionGem]

This was very helpful, Gene. Thank you. Is it conversely true that a higher crown with a smaller table can add sparkle?

Yes, generally a high crown and small table will increase the sparkle and dispersion of the stone.

 

[MissyBeaucoup]

Thanks, Gene. That’s good to know. I sure am enjoying the sapphires I got from you!