Colored stones mm surface area per carat ranges

[Nicrez]

Odd question, but does anyone know about what mms are decent for a colored stone within certain carat ranges?

Such as 1 ct could be acceptable from __mm to ___mm of surface area. I am looking around for some colored stones, and I understand the totally different ranges of size compared to thier carat weight due to cutting for color and such, BUT is there some sort of AVERAGE or range?

I am basically looking for a 2ct to 3ct equivalent stone, so I want to know what surface area to look for. I have seen some monsters cut really deep mostly, so they face up like a 1ct but really cost you more like a 2ct price... Just looking for guidelines... all the stuff GIA DOESN''T teach you. Stuff to buy in the real world.

 

[valeria101]

Welll, it is not just a matter of "monster cut" but also lower RI - if one wants to avoid windows, the cut needs to be rather deep. Of course, the same cut optimization done for diamonds can be (and is) done for any other material and shape.

If you want outmost precission for estimating spread, than there are few shortcuts to a full blown cut analysis and chances are no one but cuters did it.

Practically, the spread of princess cut diamonds makes a decent indicator. For me, about 70% depth gives a sensible guess. Off from there, this is a case for the like of GemAdviser

Anyway, I don''t know what choices you were facing. However, considering spread assumes those volumes are moulded out of shapeless putty priced by weight. In fact, the optimization of spread starts with an irregular chumk of rough often of frustrating shape that might refuse to match market expectations about what shape gemstones should ideally be. Hence the variation of price with shape - round emeralds and emerald shaped sapphires will never be the norm...

As much as I know.

Am I missing the Q ?

 

[widget]

Also...don''t different gemstones "weigh" more than others?

Is "specific gravity" the word I''m looking for???

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[Richard M.]

Date: 6/15/2005 8:23:30 PM
Author: widget
Also...don''t different gemstones ''weigh'' more than others?


Is ''specific gravity'' the word I''m looking for???

widget

As usual you''re not confused. There are two factors to consider and specific gravity (or density) is one of them. The other is cut, as Nicrez has suggested. (I''m talking about rounds here; things can get a lot more complicated with barions and other shapes).

I''d have to do a bit of research to provide specific size guidelines as requested but a s.g. table, available in nearly any good gem reference, and a pocket calculator should be a good way to begin. Take sapphire and diamond. Sapphire has a density of 4.0 while diamond is 3.52. All things being equal with the cut (which they won''t be exactly because of differences in refractive index which can affect cutting-- another time!) a 1 carat diamond should be very roughly as much as 1/8th larger in diameter than a sapphire. Diamond is less dense so it takes more of it to weigh the same as sapphire, just as it takes more styrofoam than lead to equal the same weight.

The other main variable is how the stone is cut. Sometimes stones are cut with deep pavilions or extra high crowns just to add more weight. That''s because there are "magic" price breaks at various weights, so if a cutter has a crystal that can be "cheated" to cross one of those breaks it''s often done -- more $$$. Such stones are badly cut and are smaller in diameter than well-cut stones of the same weight.

It''s a lot more complicated than this but that''s the general idea.

 

[Nicrez]

Thanks for the responses!!!

I totally understanding the differences in SG. A 2ct Hematite will be much smaller than a 2ct Fluorite, BUT, with a lot less precision, I am just hoping to find a general guideline for appearances sake to stick to. Looking at some cutters in Thailand, they like to pad on the bulk of a sapphire where it is necessary, either for color zoning, windowing on purpose, or even just the magic carat weight issue, like Richard mentioned...

Say we take a series of spinels (all same SG, so that''s constant), and we make the shape cushion cut. Basically we will also hypothetically say that the stones are all cut from similarly zoned rough, so let''s not factor that in. A 2ct with a deep pavillion would have about what surface area range? A cushion cut spinel of the same likeness as the other will have a more proportioned pavillion, so it would have what type of surface area range?

This is all just an attempt to quantify an average or standard to stay within when buying stones. I know about the variables, and would consider them as they come up, but generally would love to have just a basic guideline of what mm range I am looking at when I say I want a 2ct cushion shaped spinel. Am I looking at similar surface areas of a cushion shaped diamond minus the difference in SG as a proportion, or more like instead of surface area, consider the proportions like on a princess (as they are bottom heavy). But then proportions would not really give me the surface area and approximate life-size which I was hoping to find, unless i studied princess'' correlation of proportions to surface area...

Don''t make me do math. It hurts...

Also, there is a wholesale pricing guide for colored gemstones we saw once at GIA, but for the life of me, I can''t remember the name. Anyone know what it is? Please feel free to PM me if you know how I can purchase it... I may want to start purchasing stones very soon, and hopefully on my next trip out I will find something I consider a good deal...

 

[bar01]