A good native cut for retaining color ?

[PrecisionGem]

Good points Michael. The thing is you need to compare equal materials. I think Richard Wise is looking at his typical very high end stones, and thinking,'' hmmm these "native cuts" look pretty darn good'', and I bet they do. Obviously there are well cut native stones, and more so in the higher end stones. ($2000 and up).

What isn''t a fair comparison is to compare a $15,000 stone to a $200 US cut precision cut stone, and say look the color on this $15,000 is better than the precision cut stone, therefore native cutters know how to cut for better color.

 

[T L]

Date: 10/18/2009 12:41:46 PM
Author: Michael_E

Date: 10/18/2009 12:16:14 AM
Author: tourmaline_lover
Gene does your software take into account the natural extinction in a stone? Not all material is identical, as we very well know, and in order to cut it to provide maximum light return and color intensity, the stone''s table may have to be cut along a particular axis, or larger facets may bring out more of the sparkle from a more extinct type of material. To me, that''s where the lapidary is more of the artist, and where the computer cannot always help. I could be wrong, unless you have the software that can perform that type of analysis as well?

Extinction can be caused by only three things which are:

1. A closed axis such as in tourmaline. If light gets directed along that axis, it''s a goner and there''s nothing you can do about it except try to keep it going in the right direction. This is really the only ''natural'' extinction.
2. A deeply colored stone which is cut in a manner that causes so many bounces within the stone that the light is sucked up by the deep color of the stone.
3. Some directions in all cuts will bounce light off in a direction which keeps it from returning to your eyes. This can be ''leakage'' out the pavilion or a light path which goes out the crown at a very low angle.

Another BIG hitter here is head shadow which is the light which is blocked by your head while you''re viewing the stone. This looks like extinction, but isn''t, it''s just that there''s not enough light coming from that direction and so the stone looks dark in some areas. In Gene''s example, there''s no way to distinguish between head shadow and windowing by the program, (experience and knowing the angles can tell you this).

The other big hitters are inclusions which can scatter light inside the stone and fluorescence which can cause the stone to give off visible light in all directions. Rubies and sapphires which have micro inclusions or strong fluorescence can be cut nearly anyway and still look very nice since much of light coming from the stone is covering up any extinction or head shadow.

As far as I know there is no software which can model different light transmission rates on different axes of a stone. Some types of software can allow you to model ''attenuation'' or the amount that light is absorbed on every axis and this is handy for modeling deeply colored stones and novel cutting methods or carvings. The downside is that some of this software is pretty spendy and requires a lot of computer time to get accurate images. Most cutters either can''t afford the software and time to learn it or can''t afford the time to model most of the stones they''re cutting and so just design their cuts based on experience.

Thank you Michael. Very imformative.

 

[chrono]

Date: 10/18/2009 8:32:32 PM
Author: Michael_E
I''ve been looking at this and the ''tilt window'' thing very closely over the last couple of days and Gene is right about reducing tilt windows via higher crown and smaller tables. It turns out that you can reduce this further by cutting the crown and pavilion at just the right ratios in the Portuguese cutting style, (I''ve only been looking at round cuts, but others should be similar). Basically a well done ''native'' cut is similar to the Portuguese style, except for being step cuts and not alternating index cuts. Anyway it turns out that the steep pavilion facets combined with mutiple tiers of facets reduces tilt windowing a great deal, BUT at the cost of having perimeter of the stone being dark. Typically you don''t notice that dark perimeter so much since you''re usually moving the stone around and when you tilt it the perimeter lights up dramatically while the tilt window is rather small and not as noticeable in comparison.

I do think that it''s rather rare to find the right combination of crown height, pavilion depth and number of tiers in a native cut stone, just because it takes a lot of time to get all these things just right and that''s a luxury that most third world cutter don''t have, (not to mention equipment doesn''t lend itself to repeated cutting accuracy). On the other hand, I do think that they can do a pretty good job if they have a day or two to mess around with a high end stone...their employer knowing that a decent cut AND good weight return will maximize their profit. I think that this discussion has some disagreements mostly because many people are talking about two different things. Gene''s talking about slam-bam cutting and those of you who''ve seen nice native cuts are talking about stones which were probably higher end and had more time lavished on them, (or the cutter got a nicely shaped piece of rough and just got lucky in being able to shape it well).

You are correct about the comparison of equal material. When I’m talking about native cuts, they have different cut categories for me ranging from very good all the way to exceedingly poor. However, my comparison isn’t judging 2 stones of extremely different value but of similar pricing.

 

[serenitydiamonds]

Date: 10/18/2009 9:20:20 PM
Author: PrecisionGem
Good points Michael. The thing is you need to compare equal materials. I think Richard Wise is looking at his typical very high end stones, and thinking,'' hmmm these ''native cuts'' look pretty darn good'', and I bet they do. Obviously there are well cut native stones, and more so in the higher end stones. ($2000 and up).


What isn''t a fair comparison is to compare a $15,000 stone to a $200 US cut precision cut stone, and say look the color on this $15,000 is better than the precision cut stone, therefore native cutters know how to cut for better color.

I agree with you that with low quality or off color rough a precision brilliant cut is far far superior. However for top quality rough I feel a precision step based cut (emerald cut for example) yields the best expression of the stone''s color. This is definitely my personal opinion and I''m sure others feel differently.

--Joshua