asscher question

[waterlilly]

another newbie gem stone question:

Providing I like the patterns I see in an asscher cut gem, is there a general guideline for the dimensions of this cut? I saw one that was 1.92cts and the dimensions were 6.7 x 6.7 x 4.2. Is there a sort of quick & dirty formula for ct size and what proportions the dimensions should be for a gem that will perform well?

 

[LD]

I''m sure one of our cutters will be along to respond but in the meantime here''s my non-expert view on this!

Diamonds (and Asscher cut diamonds) definitely work to a mathematical equation. If you go into the diamonds section of the forum you''ll see that angles etc are critical if you want the best performing diamond.

You can argue that it''s the same for coloured gemstones BUT in all honesty, coloured gemstones perform differently and it''s more about colour, clarity etc. All asscher cuts, by the nature of the cut, need to be deeper than you would see in other cuts and there are good and bad asscher cuts available but I certainly would never, if I were to buy a coloured asscher gemstone, be asking the cutter about dimensions (other than wxhxd) because it wouldn''t actually tell me anything. Buying an asscher coloured gemstone is very much like buying any other coloured gemstone, you use your eyes to assess it''s beauty rather than the maths.

Not sure that helps at all?

 

[zeolite]

In optical terms, asschers are rather difficult to cut. By this, I mean your choice of pavilion angles is VERY limited (the pavilion is the bottom half of the gem).

If you go into the diamonds section of the forum you''ll see that angles etc are critical if you want the best performing diamond. They are much, much more critical in colored stones, since the refractive index is so much lower.

The critical angle is the shallowest angle that you can cut, and still prevent light from leaking out of the gemstone, and is directly related to the refractive index.

In a diamond, you must fit 5 rows of facets in a range of 25 to say 53 degrees. Thus you’re allowed 7 degree increments (25, 32, 39, 46, 53). In quartz, you must start at 40 degrees, so you have only 3 degree increments (40,43,46,49,52). A difference of only 3 degrees is VERY difficult to see when you are cutting. This is simplified, since the windmill facets must be shallower still, and still not window.

But the total depth is the sum of both the pavilion and the crown height. Thus a cutter could make the pavilion too shallow and show a bad window, and yet make the crown too high, and still meet some “correct” width to depth ratio.

So as LD says, viewing the gem with your eyes will tell you if the cutting is good. A formula is not very useful, since there are too many variables.

 

[LD]

And if you want to see one of the finest asschers (in my opinion) perhaps we could kindly ask Zeolite to post another picture of his drop dead gorgeous "please marry me" Tanzanite asscher that has to be the most heart stoppingly beautiful gemstone of 2009 for me.

 

[Michael_E]

Date: 12/30/2009 2:15:57 PM
Author: zeolite

In optical terms, asschers are rather difficult to cut. By this, I mean your choice of pavilion angles is VERY limited (the pavilion is the bottom half of the gem).

Zeolite, while I respect and agree with your viewpoint somewhat, I would have to disagree with this to some extent. The difficulty in cutting color is that there are so many different variables in terms of color saturation, zoning and R.I. Once you have a handle on how these things interact it becomes much easier to cut in any style that suits you. In the case of Asscher's, they are actually easier than stones with lower symmetry, such as pears or marquises. For stones of low R.I., you are correct in that pavilion angles are limited, but for stones with higher R.I. your choice becomes much greater. I routinely cut stones which have very moderate R.I. such as the aqua pictured,(sorry about the focus...I was a bit rushed and hand holding this outside), which have pavilion break facets approaching 60 degrees.

They are much, much more critical in colored stones, since the refractive index is so much lower.[/p][/quote]

I would disagree with this as well. The reason is that what is considered top performance in a diamond would not be top performance in color, unless that color was pretty highly saturated. In most colored stones, brightness is not the highest priority and fire does come into play. There are always exceptions, but for most colored stones having the most intense color and acceptable brightness are preferred. This naturally leads to having much higher crowns than one would have in a diamond, (the exception being much lower crowns in the case of highly saturated materials).

The critical angle is the shallowest angle that you can cut, and still prevent light from leaking out of the gemstone, and is directly related to the refractive index.

This is only true in reference to the angle of the light path and not to the axis of the gem, except when both entry and exit light paths are through a flat table. What is even more important for good reflectivity is that the angles of the light that is entering the table and exiting to your eye are such that they minimize the reflection of your head and draw their light from the area to the side of your head. These angles are a bit dependent on crown angles and height, as well as table size and are generally substantially larger than the critical angle of the material. Since the object of good cutting in color is to get the best color AND reflectivity from a stone a cutter makes compromises to achieve that, which in some areas of the stone can include cutting below the critical angle of that material.

In a diamond, you must fit 5 rows of facets in a range of 25 to say 53 degrees. Thus you’re allowed 7 degree increments (25, 32, 39, 46, 53). In quartz, you must start at 40 degrees, so you have only 3 degree increments (40,43,46,49,52). A difference of only 3 degrees is VERY difficult to see when you are cutting. This is simplified, since the windmill facets must be shallower still, and still not window.

There are two kinds of windows, the direct view and tilt windowing. For direct windowing all that is required is to cut most of the facets close to the culet above the critical angle. If the radial, or index angle is sharp enough, any windowing in smaller, separated facets, such as the diagonal facets in an asscher, are not noticed when cut below the critical angle, as they will blend into the dark areas when viewed with both eyes. One of the areas in cutting that allows one one some leeway is that the viewer usually is viewing with two eyes and their brain will blend what is being seen through both eyes in their brain. For tilt windowing viewed through the table the ONLY thing that matters is the angles of the stone around the culet. This window can be reduced by making the table smaller and by making the crown higher to some extent, but it is always going to exist and can not be removed, since it's built into the material and not a function of cutting.

But the total depth is the sum of both the pavilion and the crown height. Thus a cutter could make the pavilion too shallow and show a bad window, and yet make the crown too high, and still meet some “correct” width to depth ratio.

This is absolutely the case. When looking at "the numbers" a person needs to not only see the total depth percentage, but also the crown and pavilion depths independently. The angles of the facets around the culet, (or a good picture), is also required if one is going to have a good idea of how the stone will look when on one's hand. It is possible to have a deep belly that gives adequate pavilion depth and still have culet facets which are too shallow. This, by the way, is the ideal pavilion for a re-cut, as it usually only takes a small amount of material removal to close the window and brighten the stone without reducing it's color saturation too much.

So as LD says, viewing the gem with your eyes will tell you if the cutting is good. A formula is not very useful, since there are too many variables.

Absolutely. If a formula was available it would have to take into consideration all of the variables that affect how a stone looks AND also personal preferences. This is one of the problems with the grading of diamond cutting in that people have varying tastes as far as what they perceive as "good cutting". While some things like distinct windows and glaring inclusions are obviously unacceptable to most viewers, other things like the "quality" of color are more personal and so not open to inclusion in a formula.

Oh, just to explain this aquamarine a bit. It was cut to maintain it's color, since it's saturation was moderate. The crown is tall and steep having a break facet angle of almost 58 degrees. The pavilion is likewise deep with a break facet angle of almost 60 degrees. The total depth percentage is just over 87%. The table on this stone was made quite small to minimize tilt windowing. This picture is a bit misleading since it was taken outside with a clear blue sky which shows the stone as being more blue that it actually looks under most conditions, (that's the problem with pictures, is the color the stone or is it what the stone is reflecting ?)

 

[zeolite]

Date: 12/30/2009 2:28:48 PM
Author: LovingDiamonds
And if you want to see one of the finest asschers (in my opinion) perhaps we could kindly ask Zeolite to post another picture of his drop dead gorgeous ''please marry me'' Tanzanite asscher that has to be the most heart stoppingly beautiful gemstone of 2009 for me.

Thank you, LD. The fact that nature provided such a fine crystal, had a lot to do with its success.

 

[T L]

Zeolite,
Where's the flower picture of that gem?

 

[zeolite]

Date: 12/30/2009 9:27:55 PM
Author: tourmaline_lover
Zeolite,
Where''s the flower picture of that gem?

This one?

 

[T L]

That''s gorgeous, but there was one where it was more up close, embedded in a flower I think. Thanks for that shot though, nice!! I see the color shift a little.

 

[Arcadian]

^^^ thats some purpley goodness right thur!!

-A

 

[zeolite]

Date: 12/30/2009 10:30:03 PM
Author: tourmaline_lover
That''s gorgeous, but there was one where it was more up close, embedded in a flower I think. Thanks for that shot though, nice!! I see the color shift a little.

O.K. last Tanz flower picture, where the petal is crushed by 19 cts of Tanzanite

 

[T L]

THAT's the picture. It reminds me of art deco heaven.

Thanks Zeolite!!

 

[LD]

Oh Zeolite thank you so much for posting up the photos again. W

W That honestly makes my heart beat faster. Unfortunately, after years of collecting, that doesn''t happen often! Nice to know there''s life in the ticker still!!!

Now then, if you ever want to part with it (or marry me which might be less painful because we can then share) ...........

 

[chrono]

Zeolite,
Amazing tanzanite, amazing cutting and amazing photography. Nuff said.

 

[PrecisionGem]

As Zeolite pointed out, Asschers are a pain in the neck to cut. I''d rather cut a pear my self. ONe thing I do when cutting an Asscher or another design that has a step cut is to draw a line on the facet before cutting the next step with a marker. A Sharpie works well. This way you can see the face edge cutting in. As Zeolite mentioned, when the facets are only a few degrees apart it becomes hard to see the edge. In this tourmaline, the first tier is at 43, and the one being cut is 41. You can see I have a little more to go before it''s at the half way mark.

As a side note, Diamonds and other high refractive index stones are much more forgiving with the angles used to cut. It''s almost impossible to cut a piece of CZ and not have it look nice.

 

[MakingTheGrade]

Mmm..that tanzanite is from another world!

 

[zeolite]

Thank you LD, TL, Chrono, MTG for your kiind comments. Gene, I''ve already remembered your very important Sharpie tip.



Date: 12/31/2009 8:17:36 AM
Author: LovingDiamonds

Now then, if you ever want to part with it (or marry me which might be less painful because we can then share) ...........

LD, I had to consider your suggestion seriously. I haven''t commented on the total count of my collection, but it is well into the hundreds of gems. Plus over one hundred more crystals that have not yet been cut. If we pooled our two collections, we could give the Smithsonian Museum some serious competition!

I discussed this possible union with my wife (she''d have to go), but she was a bit less than enthusiastic about the idea.

 

[LD]

Date: 1/1/2010 3:26:08 PM
Author: zeolite
Thank you LD, TL, Chrono, MTG for your kiind comments. Gene, I''ve already remembered your very important Sharpie tip.




Date: 12/31/2009 8:17:36 AM
Author: LovingDiamonds

Now then, if you ever want to part with it (or marry me which might be less painful because we can then share) ...........

LD, I had to consider your suggestion seriously. I haven''t commented on the total count of my collection, but it is well into the hundreds of gems. Plus over one hundred more crystals that have not yet been cut. If we pooled our two collections, we could give the Smithsonian Museum some serious competition!

I discussed this possible union with my wife (she''d have to go), but she was a bit less than enthusiastic about the idea.

Oh Mr Zeolite, my husband is distraught! He was finally hoping to get rid of me! Oh well! Never mind!

Do you think we could charge an entrance fee to view our combined collection?

 

[zeolite]

Yes, and then we''ll use the entrance fees to buy more gems! Is this an addiction?

 

[Lady_Disdain]

WOuld the new couple consider adopting me? My gem collection is not huge, but I promise to look after the inheritance very well!

 

[LD]

Date: 1/1/2010 11:07:34 PM
Author: zeolite
Yes, and then we''ll use the entrance fees to buy more gems! Is this an addiction?

Need you ask? Of course, that goes without saying!!!

Now repeat after me ...................

I am a bling addict
I am a bling addict
I am a bling addict

LadyD - I''m sorry but Mr Zeolite and I intend to live forever so there will be no inheritence issues!