Mm & ct measurements

[Blue Eyes]

I''m quite mystified on how to relate millimeter and ct. measurements with how well a colored gemstone is cut, or not cut, for that matter.

For example, this is a description on ajsgems.com:

Weight:1 pc. - 4.86 cts. Size: 9.5 x 12.1 x 5.4 mm
Shape: Oval Cut: Step Cut
Clarity: Loupe Clean Enhancement: Heated
Origin: Tanzania
Lot #: SGN-00005 Stone Price: $453.


Is there a way to use these data to make even some broad assumptions on how well the stone is cut?

 

[valeria101]

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On 1/1/2004 11:29:31 PM Blue Eyes wrote:



Is there a way to use these data to make even some broad assumptions on how well the stone is cut?
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No.

Colored stones are primarily cut to show off the color, and then, if possible, to insure some sparkle. Usually, it is hard enough to play color to it's best advantage, and light return plays third fiddle. For example, extra depth commonly enhances the apparent color of less saturated rough, while shallow cutting opens up overly dark rough. Crystals with perfect color to allow the cutter to optimize light instead of color are so far between, that I am not sure what is the practical worth to discuss light return in colored gems.

Of course, maximizing weight also plays its role to detract from optimal cuts, just like it does in diamonds.

Conclusion? Surely, top quality means top color and clarity of the material itself. Some stones achieve beauty through controlled cutting and command high prices. Low quality rough cannot be made to look good (or as expected from the material: how about green Tanzanite ?!) no matter how much effort ones dedicates to cutting, and prices reflect this.

Can light return AND color be maximized? Sure: one can probably fill a good grocery bag with a year's production of any given type of precious rough good enough to allow this. However, there is software which allows to model and evaluate light return in non-diamond material. Not sure of the specs, but GemCad and a (mysterious to me) product of OctoNus does this, among others. I am quite curious myself what is the market for cut improving software in colored stones...

 

[strmrdr]

Val or others can correct me if im wrong but the way I see it can be summed up this way:
Start with a good grade of rough:
5 classes of gemstone cut:
1: cost hundreds to thousands of dollars in cutting fees having the best cut it taking everything into account and sacrifice weight.

2: cost hundred - hundreds getting the best the stone has to offer but not going all out.

3: cost 10s of dollars - having it cut fast and efficiently with an eye towards retaining weight but put the work into it to get a decent finished product.

4: automated cuts or partially automated cuts - cost - I don’t know but not much.

5: cost 28 cents - just enough cutting so it looks something like a gemstone.

My costs may be off but Id be surprised if the ratios were that far off.

Most cut gemstones available will fall into the 3-4-5 with 5 being far more common.
Calibrated stones are often 4.
1s are extremely rare because most people don’t want to pay for that level..

Ajs has a reputation for selling stones in the 3 range from what Iv been able to gather.

 

[valeria101]

Thanks for the challenge!

Al this depends alot on what stone we are talking about. Extra weight is sometimes desirable for one reason or another (one needs depth to insure both sparkle and depth of color in some material with rather weak optical properties, so much so that optimizing either will make the piece unwearable, etc.) The war with weight is all there, of course, but weight ithself (actually the depth of cut) is not as much as an absolute evil as it is in round diamonds.
Also, cheap methods of cutting may yield very nice results, esp for the cheaper rough where wasting material rather than cutting time is not an issue.
Not to mention that "native cutting" is nowadays producing lots of great surprises, so there is no reason to dismiss a stone sight unseen just because it wasn't cut in the US or germany.

AJS? Their best stones have precission cuttin, and some of the cheaper too... I am not sure AJS cuts all it's gems. The one thing about them: there are no too many stores which stock both $10 white beryl and >10cts tsavorite in the same place. And this brings another point: the bets stones 9an dthis means larger of good quality) do receive the best cutting. This is something that does not fit with the theory that weight retention is the greatest concern and greatest curse of gem cutting.

I would only attempt to clasify cut categories for one type of gem at a time. However, I believe that this exercise leads one to inevitably miss great cuts and great gems that happen to fall in the wrong category. Also, cost is surely not the only constraint to be taken into consideration: in some cases, the ideal material which may allow optimized cutting is simply not there! Rarity )or rather scarcity) of rough should be taken into account - one more argument to discuss this gem-by-gem.

All in all: I think that some tutorial on colored gem cut may complement Pricescope well, even an informal one.

 

[Blue Eyes]

Thanks for the replies, both of you!! Years ago (in my mid 30s's when I was making a 6-figure salary) I bought stones and had pieces made like a crazy woman. I knew very little about buying colored stones but I put all my trust into a high-end jeweler in Birmingham, Michigan.

I told him the gemstone I wanted, he got a selection of loose, and I picked using only my untrained eye. What I mean is, if I liked the color, size and cut ... I bought it !!! Fearless? Yes. For my work I have several tanzies, rubies, green tourmaline ... the list goes on.

Flash forward, 4 years or so, I am back in the market for loose, and since I've been researching, I've never been more anxious about making good choices on what to buy, and from whom ! This site is fabulous. I do think, however, that I have read enough here and on eBay about ajs to at least give them a test run. I am OBSESSED with finding a cushion cut tanzie 4 ct or so!

I've been watching the Gem Shopping Netowrk on DirectTV and have lost many hours of sleep because of it. The channel is like a drug: I am hooked. They have only medium to high end gemstones, dealing only in colored stones. They;ve been showing mandarin garnet at about $1500 a ct, tanzies in all prices, estate pieces, Australian opal and diamond pendants ... beautiful stuff.

Happy New Year !

 

[strmrdr]

fwiw
I agree with Val my levels are an over simplification.
You can get some great cuts at the less than top levels without the price.
She is also right on that each stone has to be taken individually,

I will have to refine it a little and Im not sure it will be that useful but it does represent at least roughly what’s available. :} (Pun intended)
Thanks Val.

 

[valeria101]

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On 1/3/2004 9:45:00 AM Blue Eyes wrote:

I am OBSESSED with finding a cushion cut tanzie 4 ct or so!
They;ve been showing mandarin garnet at about $1500 a ct, ...
Happy New Year !----------------

I guess my posts (and the look of the site) were the source of all those AJS calls. 4cts is rather small for tanzanite, so color saturation may be a problem (which was discussed on a recent thread here, with plenty of sources listed too). If you get 5 min, take a look at Africagems - they cary a small but very inspired selection, very much worth the trouble.

Best of luck!

 

[elmo]

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On 1/3/2004 9:45:00 AM Blue Eyes wrote:

I do think, however, that I have read enough here and on eBay about ajs to at least give them a test run.----------------

For AJS I'd personally stick to the ebay stuff...had an unpleasant experience with them trying to get information on one of their higher-end pieces. For stuff in the few hundred dollar range though I expect they're just fine.

p.s. The gem channel tanzanite I've seen on tv often has mediocre color, and they want silly prices for the stuff. The very best gem tanzanite can be had in the $500/carat range, but I thought I'd seen prices more than that on tv? And I agree with Val, it might be easier finding the best color in a larger size. I really like the color of top tanzanite, but have enough reservations about its wearability (softness and brittleness) that I doubt we'll ever own one.

 

[valeria101]

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On 1/3/2004 4:13:04 PM elmo wrote:

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The very best gem tanzanite can be had in the $500/carat range, but I thought I'd seen prices more than that on tv? I really like the color of top tanzanite, but have enough reservations about its wearability (softness and brittleness) that I doubt we'll ever own one.----------------

I have to disagree a bit (as usual...

). I guess 500$ is a good number, but great material may top three times that. One thing about tanzanite prices is that the price / carat seems to tapper down in larger pieces of 5 cts and above (the darkest, most expensive material will already look too dark and/ or is vanishingly rare in such size). The small stuff (below 3cts) of intense color seems to command the highest price per carat, and there are few stones who can hold their color is such small sizes. Not to mention that I find it hard to choose tanzanite over sapphire in these smaller sizes & high price. Strange ("collection") items such as green, teal blue or sapphire-blue colors and untreated pieces get some baffling prices. All in all, large pieces of awesome color touch $800 and I do start wandering below 400$ for anything marketed for US buyers.

Is it brittle? Not really: isn't tanzanite comparable to toumaline in this regard? Not good for tossing around: but I would not dismiss it. One may get facet joints worn out with wear, but the material is harder to chip and splinter than diamond and plenty hard for "ocasional wear" jewelry.

AJS communication... my Qs about their former encounter with double-star rubies were welcome. Buying a blue spinel from them was a good experience. They seem to have at least one experienced gemologist in their staff and informed Qs (even non-commercial) were welcome. I guess every store has it ups and lows.

 

[elmo]

You're right, as the size goes up the price goes down. That's an interesting phenomonon for gem pricing, isn't it

, even if understandable for tanzanite. I guess I think of a "large" stone as something over 15 carats or so

. Heck, there are people posting Richard Homer-cut gem-color tanzanite in the 7-10 carat range at around $800/ct, so I think that's too much to pay.

I'm pretty sure that the heat treatment that virtually all blue tanzanite receives makes it fairly brittle, and that's in addition to it being soft (softer than the quartz that makes up most of the dust that's around, according to Richard Wise's new book). They make me think of pearls, something that might not be around for my grandkids to wear.

Don't take my word for it, though, I'm not a gemologist

.

 

[valeria101]

Here are sone stats:

Pearl Hardness: 3.5 - 4.0

Opal Hardness: 5.5 - 6.0

Tanzanite Hardness: 6-7

Tourmaline Hardness: 7-7.5

Sapphire Hardness: 9

If you like the color of tanzanite, no need to avoid it. Heating does not make (surviving) gems more brittle (actually quite the contrary, at least in corumdum).
Mohs is not a liniar scale (nor is it exponential, but rather has some structural gap before Corumndum's position (see below).