New Mineral Named After GIA’s John Koivula
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 Post subject: Cutting a piece you don’t want to cut
PostPosted: Sun Oct 07, 2018 3:29 pm 
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I really just want someone to tell me what to do with this. I’m good at not returning rough bought on the internet because.. I don’t know, it’s just easier to stow it in a box and tell myself I’ll figure out something to do with it some day than telling someone “I don’t want what you sent me.” I’m not buying any rough over the internet anymore... but that’s beside the point. What do I do with this piece of topaz? What would you do? Bad pic, but I think shows the problem. Depth is about 7mm, LxW 18 x 14mm appr dims. I have looked for a design but nothing is jumping out at me :(
Mainly because I am thinking about all the grinding I’m going to have to do....


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 Post subject: Re: Cutting a piece you don’t want to cut
PostPosted: Sun Oct 07, 2018 7:16 pm 
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It doesn't look like a particularly difficult piece of rough to me. The two dimensions that matter are the depth as a proportion of width. The thin side of the stone is the same side as the notch. By the time you pre-form the stone on that side, you will have dealt with the biggest part of the notch problem.

Let's say you keep close to all of the length, and shape the width down to the point that the notch is not an issue my guess is that you finish with a cushion shaped preform of about 10 X 16 X 6.5. Plenty of ways to cut a cushion with a +-60% depth and no window considering the Critical Angle of Topaz is approximately 38 degrees.

I don't know what you mean by too much time grinding. Pre-forming before you dop should take only a couple of minutes.

I would anticipate a yield of between 33% and 40% on this rough unless something shows up that I can't see from the picture.


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 Post subject: Re: Cutting a piece you don’t want to cut
PostPosted: Sun Oct 07, 2018 10:28 pm 
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I'll try. I haven't had much success pre-forming before dopping. Usually ends up being more wasteful due to my hand not holding the rock steady as it would've been if dopped. I'll try to fix that notched edge by doing so this time though. My usual modus operandi is to look for an existing design that fits the rough at hand dopped to its center of mass, and grind away what doesn't fit with 325 or more usually 600. That can take some time. Don't have a way to create a design currently either, which further inhibits the process that you describe. Will check back with preform dims...


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 Post subject: Re: Cutting a piece you don’t want to cut
PostPosted: Sun Oct 07, 2018 10:40 pm 
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micra wrote:
I'll try. I haven't had much success pre-forming before dopping. Usually ends up being more wasteful due to my hand not holding the rock steady as it would've been if dopped. I'll try to fix that notched edge by doing so this time though. My usual modus operandi is to look for an existing design that fits the rough at hand dopped to its center of mass, and grind away what doesn't fit with 325 or more usually 600. That can take some time. Don't have a way to create a design currently either, which further inhibits the process that you describe. Will check back with preform dims...



I can guide you through pre-forming, doping, and cutting. We don't need no stinking patterns. LOL. We will let the stone tell how it needs to be cut.

This stone is so close to being a pre-form that it will be easy and quick.

Since this is Topaz we will want to make sure that the Table is not parallel to the cleavage plane. About 7 degrees off should work fine.

What kind of machine do you have?

Do you have a target dop for your transfer jig?

Where do you live?


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 Post subject: Re: Cutting a piece you don’t want to cut
PostPosted: Fri Oct 12, 2018 5:41 pm 
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Why preform unless it is large rough with obvious waste that needs to go and you are doing it with something other then a faceting lap.
Grind a false table about where you think the table for the finished stone will be and dop it up and let the "pre forming" set the size. With large rough set an angle that is close to some of the bigger facets on the pavilion of the design and grind away with a rough 180 or 360 depending on the size and type of rough...just stop well short of the finished facet.

By doing that you can also easily see if you need to reheat the doping wax...yes doping wax... and nudge the stone one way or another to maximize yield.


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 Post subject: Re: Cutting a piece you don’t want to cut
PostPosted: Sat Oct 13, 2018 12:07 am 
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You can dop a preform almost perfectly using a target dop. I have found this results in the highest yield for me. Not saying others don't have a system that works better for them. Pretty much all the profesional cutters I know cutting very high end gems all agree. You make you most money when you preform, not when you cut.


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 Post subject: Re: Cutting a piece you don’t want to cut
PostPosted: Sat Oct 13, 2018 9:52 am 
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works for me.. I was pre forming and read an opinion that made sense to me and switched...like it better


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 Post subject: Re: Cutting a piece you don’t want to cut
PostPosted: Sun Oct 14, 2018 8:17 pm 
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I need to preform so that I can see what I have to work with. Just can't visualize the final shape with that extra fat on there, not knowing where the actual center of mass is. Even just trying to grind a flat spot I find I need something other than just the rock to hold onto. So far I have dopped and ground a table, dopped onto the table and have ground away that edge with the big chip in it, ground a rectangular shape and determined target L/W, selected a design, and now just need to nudge the rock's position on the dop (I have only ever used wax, so no worries there) and start cutting. I could not see what to do until I got it preformed, then it was pretty straight forward. I don't play chess well, either.


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 Post subject: Re: Cutting a piece you don’t want to cut
PostPosted: Sun Oct 14, 2018 9:56 pm 
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Your pre-forming system on a dop is slower than hand pre-forming, but if that is what works for then do it. No extra loss of weight will occur because of this.

However, pre-forming to a L/W ratio that matches a diagram is where excess loss occurs. This is one of the important differences between those who facet, and those who cut gemstones.


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 Post subject: Re: Cutting a piece you don’t want to cut
PostPosted: Mon Oct 15, 2018 4:41 am 
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1bwana1 wrote:
However, pre-forming to a L/W ratio that matches a diagram is where excess loss occurs. This is one of the important differences between those who facet, and those who cut gemstones.

Faceters don't cut gemstones??? People facet gemstones for various different reasons and markets. Judicious choice of a diagram takes the guesswork out of producing a stone, and allows the weight recovery to be calculated before you even start. Cutting on the fly may give you greater flexibility in matching stone to rough, but it requires greater experience, or experimentation, to choose the correct index settings, angles, and tier increments to produce a pleasing shape, regular facet placement, good weight retention and optical performance. It's not every gemstone cutter's cup of tea.


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