Face Up Size Of The Stone

[pwsg07]

Occasionally, I have seen description of the stone mentioning that the face up size is large for the stone of the same carat weight. How can I tell the face up size of the stone is large or small for stone of the same weight? I am looking for stones cut with good proportion. How can I tell if the stone if too shallow or too deep just by the dimensions of the stone? Thank you.

 

[chrono]

http://www.brilliancejewelry.com/Gemstones/conversion.html

 

[PrecisionGem]

The answer really is, you can't by the dimensions. You can by looking at the stone. Colored stones are more complicated than diamonds, and there are many more types of cuts, so there is no ideal ratio of depth the width etc. Some designs have very high crowns, and shallow pavailions, yet perform very will, other it's just the opposite. Each gem type has it's own refractive index so a cut that would have proper angles or proportions for say a sapphire, would not work for an amethyst.

 

[normanintheskywithsapphires]

Isn't there a certain depth ratio for every gem variety that is considered 'good'?

 

[PrecisionGem]

Isn't there a certain depth ratio for every gem variety that is considered 'good'?

No, its a function of the design and the refractive index of the stone.

 

[chrono]

For non-precision cut stone, you might be able to hazard a good "safe" guess but not when it comes to precision cut stones due to the design.

 

[normanintheskywithsapphires]

No, its a function of the design and the refractive index of the stone.

But does not every gem-variety have only one refractive index only? Does that not mean there is an optimal for sapphire, one for tsavorite, one for etc.?

 

[mastercutgems]

Norman;

There is a single "rule-of-thumb" most cutters go by and that is the 75% ratio depth to width. That would be buying rough to cut or a gem to re-cut... As most cutters know over 90% of all cuttable minerals have critical angle below 42 degrees. That being said unless you tend to go with super deep pavilions or super high crown designs you will easily fall into proper cutter angles and optical appearance with that ratio...

Of course you can play with cuts all day long and it will depend on many things other than critical angles such as RI; DR to SR minerals, depth or saturation of color, inclusions, etc.... So many are a case by case situation; but to paint the entire means of faceting with a ratio for optical performance; the 75% depth to width has worked well for 10K's of cutters and rough alike to get a properly optical proportioned gem...

I think on my list of minerals which is vast; opal is about the worst with a critical angle of 43.4 degrees; so that being you would want to stay somewhere in the 45 degree angle on your culet mains... I have gotten rough that worked well with .5 degrees above critical angle that still did not window; but normally I stay somewhere between 2 to 5 degrees above the critical angle. Still it all depends on other factors as well; but critical angle is extremely important when you want reflection back through the table...

Good question...

Most Respectfully;

Dana Reynolds
ASG Certified
Supreme Master Gem Cutter
#96CGE42

 

[normanintheskywithsapphires]

Dana,
OOOKay! I see the world is huge and I know nothing, but thanks for taking your time to explain to the likes like me.
If I had better eye-sight, I might pick cutting as a hobby, but one needs a good eyesight, right? Though now I see you wear specs as well....

 

[mastercutgems]

Dana,
OOOKay! I see the world is huge and I know nothing, but thanks for taking your time to explain to the likes like me.
If I had better eye-sight, I might pick cutting as a hobby, but one needs a good eyesight, right? Though now I see you wear specs as well....

Norman;

You would make a very good cutter; as you have the love of the gem... I have been near-sighted since the 3rd grade. My colleagues laugh at me all the time as when I inspect my cut I look over my glasses to see as I have great vision up close... But all that aside... that is what they make the opti-visor for... I taught faceting for over 6 years at the local gem and mineral club. I taught people from 8 years old to 83 years old and they ALL were able to produce a lovely gem worth keeping and setting...To me it is like painting by numbers Aside from matching existing facets on previously cut old heirloom gems you just follow directions, degrees, and indexes and the feel for the pressure of the gem against the lap...

You can do it and you can have a lot of fun just re-cutting gems as the rough market is kind of tough as there is much more to consider and there are so many things you think you did not see; that was there that you missed... That can make the difference between a hundred dollar a carat gem from a 20 dollar a carat gem...

But on the subject of facing up larger in gems you see a lot of it with diamonds where there is so much standardization in mm sizes to weight ... Staying close to but above critical angle in the culet area, lower crown angles, and larger table all can help make a round gem look a little bigger than a traditionally cut gem in the same weight range.

Good question as when I am buying gems like 1+ ct. diamonds I look for that as it does give the appearance of a larger gem for the money..

But most anyone can learn to facet a gem and learn to facet it well.

Most respectfully;

Dana Reynolds
ASG Certified
Supreme Master Gem Cutter
#96CGE42

 

[normanintheskywithsapphires]

Dana,

Thanks for the encouragement. I might try a beginners class somewhere before I jump and buy equipment (yes, one learns when getting older, sometimes, sort of...)
Are there cutter-classes? Like two week intro and testing? Any recommendations?

 

[mastercutgems]

Dana,

Thanks for the encouragement. I might try a beginners class somewhere before I jump and buy equipment (yes, one learns when getting older, sometimes, sort of...)
Are there cutter-classes? Like two week intro and testing? Any recommendations?

Hi Norman;

I would try searching your local gem and mineral clubs. If you have one close I am sure one of the cutters will gladly take you on as a student as it is fun and you make friends that way... Many clubs like the one I was president of had their own faceting machines, cabbing, and sphere machines to use by the members...
You get into testing and stuff like that when you go into competition cutting and want some sort of certification to see how you place in the big scheme of things. But to cut and enjoy you need no certification. I just did it as I was around no one that cut and wanted to see how I was doing...
The gentleman I got my machine from sold me the ultra-tec machine, laps, dop wax, a few pieces of quartz, andalusite, and beryl and the 3rd edition of faceting for amateurs and that was it... He would not show me anything?? So that is why I help when I can...

Thank goodness I had 8 years as a tool and die maker under my belt; so it all fell into place rather easily.

Let me know if I can help you in any way...

Most Respectfully;
Dana Reynolds
ASG Certified
Supreme Master Gem Cutter
#96CGE42

 

[normanintheskywithsapphires]

Local gem-clubs? Oh, I didn't know such are around. I will check. Thanks.