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 Post subject: questions on the nature of gem cutting and light rendering
PostPosted: Tue Oct 27, 2020 3:39 pm 
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Hello again. I have been cutting faceted stones quite a lot lately and I have been wondering if you had any thoughts on gemstone cuts. I really like making designs on gem cut studio to fit the roughs I have; I feel like it gives me more creative opportunity in the process and helps me make a more sleek design. But I am also interested in cutting without diagrams. I have cut several stones without diagrams, sort of just basing the angles on what other designs have, and have had both good and questionable results.

But even when I create a design to fit a certain rough on gem cut studio, I still see where I could have gotten better yield by cutting without a diagram. I am wondering if lapidaries who cut without diagrams have certain rules in order to make their stones look good. I say this, because on gem cut studio it seems like one degree can significantly change how a stone looks. I can't imagine traditional cutters really knowing about such changes.

Do you feel like it's better just cutting from diagrams, so you can get the angles "optimized"? It seems like it would also be easier to cut "commercial" style (or whatever term you would like) on a jamb peg.

Ok, and one last question: on the gem cut studio preview, there are areas that show up in the stone that are completely black. At first, I just though they were high contrast areas, so I incorporated them into my designs. But I think I read someone mentioning that they could be reflections of the background from the table reflecting forward. If so, they would be undesirable in any design. And connecting this to my previous thoughts, I don't see how you could really predict whether this would happen if you were cutting without a diagram/rendering software.

Thank you gemology online people


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 Post subject: Re: questions on the nature of gem cutting and light renderi
PostPosted: Thu Oct 29, 2020 4:03 am 
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rattlesnakeagate wrote:
I can't imagine traditional cutters really knowing about such changes.


Of course they do, at least the experienced and skilled ones. What makes you think otherwise?


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 Post subject: Re: questions on the nature of gem cutting and light renderi
PostPosted: Thu Oct 29, 2020 10:24 am 
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Yes they do, and in a very sophisticated way.

You make the mistake that a lot of neer cutters do in believing that the cutting is all about brilliance and reflection pattern.

It is if you consider yourself a faceter. But gem cutters view things a bit differently. Their fist priority is to maximise the beauty and value of a gemstone.

Since color is the number 1 attribute in a gemstone's value they tend to focus on maximizing that. This includes dealing with things like orientation, zoning, and managing tone.

Next is dealing with inclusions. Either by removing them when preforming, carefully orientating them, or placing them in "busy" points in the stone to minimize visibility.

Next come size. Generally larger retains most value.

An experienced gem cutter will manage all these things by choosing the right compromises within an attractive outline, and facet placements that give a good brilliance with minimal extinction, and windowing.

Experienced gemcutter's using very primitive equipment produce excellent results. They rely on years of experience, and have developed a set of parameters that they operate in. They can visualize a finished stone inside a piece of rough as well or better that anyone using a diagram or program to create one. They know how each compromise they make will affect the resulting gemstone.

The cutters job is to maximize value, if he doesn't do a good job at that he will not be employed long as a cutter.

Take a look at the award winning gemstones over the years in the AGTA Spectrum awards. Except some of the unusual categories like concave faceting, many of the winning stones would not score high on a faceters cutting criteria.

It is the difference between faceting, and gem cutting as the primary focus of the task.

You will find that if you keep your designs with a general set of parameters for pavilion angle within the diameter of the table (last two tiers ), table size, crown height, and patterns that help manage colors, that you will be able to produce beautiful stones. These parameters can be expanded or restricted based on RI of the material but you should focus on Snell's Law not critical angle.

In order to get the highest cut grade for brilliance using G.I.A.'s grading criteria you need a brilliance score of 75%. You start at 100% and deduct the percentage of windowing and extinction. The total percentages must always add up to 100%. There are other scores for things like symmetry, polish and workmanship as well. But these can also be achieved by a talented cutter even hand holding a dop in one hand, and hand spinning laps with his other. No indexing, or angle setting devices of any kind. I have seen it done many times.

Keep in mind that this is not an endorsement of the badly cut native stones. These don't maximize value, and are not attractive. Masterly gem cutting is the art of compromise to produce attractive stones with good workmanship.


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 Post subject: Re: questions on the nature of gem cutting and light renderi
PostPosted: Thu Oct 29, 2020 12:34 pm 
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Well said Steve. No need to add in other than some of the awards gem cutting lends itself to an art rather than traditional gem cutting. As you know the Idar German groups have mastered for many years.

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 Post subject: Re: questions on the nature of gem cutting and light renderi
PostPosted: Thu Oct 29, 2020 1:32 pm 
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I once asked one of the best cutters in Idar about what angles he had used to cut an amazing Paraiba Tourmaline. This was back when all Paraiba was actually from Paraiba State in Brazil. The African copper Tourmalines had not been discovered yet. The original Brazilian stones were very special.

He just laughed and said no to bother asking, because he has no idea what angles, he let the rough guide him in his cut.

I learned a lot from the Idar cutters back in the day.


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 Post subject: Re: questions on the nature of gem cutting and light renderi
PostPosted: Thu Oct 29, 2020 2:15 pm 
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1bwana1 wrote:
This was back when all Paraiba was actually from Paraiba State in Brazil.


Just like many species, Santa Maria Aqua, true Madeira Citrine and others.

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 Post subject: Re: questions on the nature of gem cutting and light renderi
PostPosted: Thu Oct 29, 2020 10:17 pm 
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Thank you bwana for your in-depth reply. Are there any guides on cutting in traditional style?

Dioptase wrote:
rattlesnakeagate wrote:
I can't imagine traditional cutters really knowing about such changes.


Of course they do, at least the experienced and skilled ones. What makes you think otherwise?


There is a thread on here called "ideal baguette cut" in which someone rendered the angles for an asscher cut. The light pattern was not very appealing using the recommended angles. So what I'm saying is, it seems some angles are not going to work well together, even if the stone seems to be a good "shape". Like a cutter may make a stone well shaped by his eyes but the interaction between the angles he chose may not be ideal. Maybe this is not as big an issue as I think it might be, or the cutters are typically putting enough gently sloping tiers and facets that it does not stand out.

I figured out the dark areas I see in the rendering are from the headshadow simulation. I am assuming head shadow is something you want to minimize in all cases?

Thanks everyone!


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 Post subject: Re: questions on the nature of gem cutting and light renderi
PostPosted: Thu Oct 29, 2020 11:19 pm 
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rattlesnakeagate wrote:
Thank you bwana for your in-depth reply. Are there any guides on cutting in traditional style?

I figured out the dark areas I see in the rendering are from the headshadow simulation. I am assuming head shadow is something you want to minimize in all cases?

Thanks everyone!


Nothing you can do in the cutting to minimize "head Shadow". This is a simulation in the program to show the reflected image of the viewers head in the stone. While it is a real phenomenon in certain lighting and viewing angles, in cannot be dealt with in cutting. Just turn it off so that it doesn't confuse your view of the stone.


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 Post subject: Re: questions on the nature of gem cutting and light renderi
PostPosted: Fri Oct 30, 2020 11:53 pm 
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I cut with no diagrams about half the time now and lately cutting a lot of emeralds so I never use a diagram. If you’re doin a step cut, no diagram is needed and you can really easily follow the rough shape for top and bottom. My last emerald i got a 61% yield and great color despite a small window that I needed to keep because the shape and weight and customers demanded it.

Even a brilliant cut now I don’t use a diagram because it’s ingrained in my head. The only thing to adjust is how shallow the pavilion angles are and that is always based on the depth of the stone vs the critical angle. Try practicing with no diagrams with an emerald cut. It’s quite fun and a good exercise. All the facets are on the 8 main numbers on a 96 index gear. Angles will be based on the rough. Good luck and have fun

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 Post subject: Re: questions on the nature of gem cutting and light renderi
PostPosted: Sat Oct 31, 2020 1:46 am 
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Justin,

Assuming you have a deep enough piece of rough, what do the critical angles of the material have to do with choosing your pavilion angles?


Round brilliants are as easy as emerald cuts.

Pavilion:
Cut the girdle facets on the 8 main index numbers and 1/2 way in between (on the sixes)

Cut the mains on the main index numbers plus 3.

Control the height of the breaks with the difference in angle between the breaks and mains. Usually between 1.5 and 2.5 degrees.

Crown:

Girdle facets on the same indexes as the pavilion.

Mains depend on whether you are cutting stacked mains or unstacked mains.

Stacked mains do the opposite of the pavilion. 8 main indexes minus 3

Unstacked mains use the 8 main indexes plus 3

Control the height of the breaks with the difference in angle from the breaks. Usually between 5 to 6.5 degrees works well.

Stars are cut on the 8 main index plus or minus 3 using the opposite side that you curb the main on.

Stars are most often between 10 and 13 degrees steeper than the mains depending on the table size you want, and the kite shape of the mains you like.

Oval, cushions, pearshapes, heart shapes, marquise, are all just deformations of that pattern that primarily depend on length to width ratios.

With step cuts the difference in angle between tires is meaningless. Do what the rough allows.

With and sort of brilliant cut (facets that are some form of diamond or triangular shape) the difference in angle between tiers control the shape. Small differences creates taller thinner facets, big difference creates shorter wider facets.

I start my students out without diagrams. Most can cut any shape and pattern without a diagram in a few months. My goal is for them to be able to look at a stone, and cut one with the same pattern.

After they reach a decent level, then we start them with diagrams as a starting point and the modifying it in their heads while cutting to fit the piece of rough they are working with.

This learning has become much quicker now that Gem Cut Studio allows them to practice quickly and easily without risking rough.


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 Post subject: Re: questions on the nature of gem cutting and light renderi
PostPosted: Sat Oct 31, 2020 3:23 am 
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1bwana1 wrote:
Oval, cushions, pearshapes, heart shapes, marquise, are all just deformations of that pattern that primarily depend on length to width ratios.

With step cuts the difference in angle between tires is meaningless. Do what the rough allows.

With any sort of brilliant cut (facets that are some form of diamond or triangular shape) the difference in angle between tiers control the shape. Small differences creates taller thinner facets, big difference creates shorter wider facets.

What I have learned from you is that cutting brilliant-style stones other than rounds without a diagram depends on well-proportioned preforms and placing the facets to follow the shape, rather than the shape being generated by the facet placement. This seems a crucial distinction between 'traditional' gem cutting and meet-point faceting.


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 Post subject: Re: questions on the nature of gem cutting and light renderi
PostPosted: Sat Oct 31, 2020 11:18 am 
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Duncan Miller wrote:
What I have learned from you is that cutting brilliant-style stones other than rounds without a diagram depends on well-proportioned preforms and placing the facets to follow the shape, rather than the shape being generated by the facet placement. This seems a crucial distinction between 'traditional' gem cutting and meet-point faceting.



Duncan,

The preforming applies to all shapes and facet patterns whether brilliant, or step.

During preforming you can manage any issues with the rough like inclusions, orientation regarding color and shape, and bulge on the pavillion. You can do this freely and precisely with minimal loss.

In the meet point system, when you do this after dopping on the machine, any losses from chasing a problem on one side of the stone must be repeated on the other side to maintain the symmetrical distance from the center of the dop. This means at a minimum you double the loss, and often more depending on the symmetry of the cut.

After you have an acceptable preform it becomes easy to precisely position the stone on the dop using a target dop, and a dental wax pick up dop. the link below is to a video of a Zoom meeting I recently did for one of my classes during the Covid-19 lockdown here. I am am a better cutter than video announcer, but you should get the point.

https://www.youtube.com/watch?v=JuMpQH6 ... e=youtu.be

If you look at the oval preform Amethyst I used in the video keep in mind that if this were an expensive stone like corundum where I was going to use the classic mixed cut faceting pattern, I would expect a 70% to 80% recovery from the preformed weight. Other patterns would likely yield a bit smaller, especially meet point style cuts.

I should also acknowledge that if the material is really valuable I don't use tha machine for managing symmetry at all. I will raise and lower the mast to place each facet precisely so that I can ignore the position of the stone on the dop. This is a slower cutting process, but increases yield another couple of percent. Only worth doing on expensive stones.

The target dop/wax pickup method works very well when dopping cut stones for repairs as well.

I hope you find this info interesting and helpful.

Steve


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 Post subject: Re: questions on the nature of gem cutting and light renderi
PostPosted: Sat Oct 31, 2020 12:30 pm 
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Steve - Thanks for the dopping video. Using a keyed target dop with orientated 'cross hair' lines is a good tip. Duncan


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 Post subject: Re: questions on the nature of gem cutting and light renderi
PostPosted: Wed Nov 04, 2020 2:48 pm 
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Hi Steve,

Thanks for posting the dopping video. It was very informative with great ideas.

Randy

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 Post subject: Re: questions on the nature of gem cutting and light renderi
PostPosted: Sun Nov 22, 2020 9:44 am 
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1bwana1 wrote:
Justin,

Assuming you have a deep enough piece of rough, what do the critical angles of the material have to do with choosing your pavilion angles?

=.


Well for example the emerald has a critical angle of 39.8 so I would definitely try to not go shallower than that if I can so I won't window the stone, but if I have enough depth, I will try to go even steeper, to at least 42 so that I won't have an immediate tilt window when looking at the final stone. On emeralds, my customers would prefer me to make a small window than make the stone width thinner to give it enough depth to close the window at 40 degrees. Usually though I would never consider leaving a window or even getting within a few degrees of the critical angle unless I absolutely have to.

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