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 Post subject: How thick is too thick for a girdle?
PostPosted: Thu Aug 27, 2020 4:50 pm 
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Not sure if this should go in lapidary, or in the jewlers bench, so I’m putting it here...

I’m currently working on cutting a rather chonky clear topaz in a modified portuguese cut so that the angles match up to topaz better than the design recommended... Anyway, the stone started at 44ct. Pavilion cut gorgeously, and now I’m working on the crown. Only thing is I don’t want to waste too much stone - I want to go for a nice hefty weight. I’d like this topaz to end >10ct.

That said, my girdle is currently sitting at 2.7mm. I’d like to be able to set this gem in a pendant — but don’t have a lot of experience with that. How thick is too thick for a girdle?

Furthermore, how much wiggle-room do I have in crown angles? If I’m going to shrink this girdle, I’d like my break facets to be steeper, but don’t want to screw with the brilliance of the gem. Logically I feel like crown angles should be - at most - smaller than the critical angle of the gem so as to let light in. I’m not certain if this is correct though, most articles talk about critical angle solely with regards to the pavilion.


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 Post subject: Re: How thick is too thick for a girdle?
PostPosted: Thu Aug 27, 2020 5:28 pm 
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What is the current stone dimensions? Lengthxwidth. Round, Oval.....etc.
Girdles are an arbitral width in my opinion. You will get different answers from different cutters and a specific jeweler has there own preferences. Typically harder stones you can go smaller as not as much chance a jeweler is going to break the stone. Then there is a setting style to be considered prongs, channel, v-tip etc.....
Also remember that the thicker the girdle the more material light has to pass thru to reach the pavilion facets and return back, sometimes a thick girdle can give you a dead looking stone.

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 Post subject: Re: How thick is too thick for a girdle?
PostPosted: Thu Aug 27, 2020 8:12 pm 
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It’s a circular Portuguese cut. Its got a diameter of 13.7mm. The pavilion (guessing here because it’s on a conical dop) is about 7mm. I’ve got 3mm of crown if I go with how it’s currently cut, and the girdle itself is about 2.8mm.

I haven’t decided exactly what I’ll set it in yet. It’s an odd diameter so I figure I’ll need to fashion my own setting.


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 Post subject: Re: How thick is too thick for a girdle?
PostPosted: Fri Aug 28, 2020 1:56 am 
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I would take it down to about .35mm after pre-polish and polish you end up between .25 and .30mm. If you do not feel comfortable going below .5mm stop at .65 and end up between .5 and .6mm. Your going to have to decide where your comfortable, do you want to leave a little larger girdle in the case you need to fix the crown for some reason as you cut the rest of the tiers.

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 Post subject: Re: How thick is too thick for a girdle?
PostPosted: Fri Aug 28, 2020 2:48 am 
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Thanks - the more I look at it the more awkward the giant girdle looks. I still want to preserve as much weight as possible though, so i wonder how feasible it is for my break facets dronbe at like 45d.

I know the critical angle of topaz is around 38d — so would that prevent light from entering via those facets in a meaningful way?


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 Post subject: Re: How thick is too thick for a girdle?
PostPosted: Fri Aug 28, 2020 6:40 am 
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White Topaz is not a particularly valuable stone. Since it is colorless and clean there is no need to worry about color management and hiding inclusions. The rules of maximizing value in colorless stones is a bit different than for stones where color is a primary component of value. In this case, The value of your finished stone will be dependant more on how brilliant it is, and how attractive the facet pattern is, than the value of any weight you might save by fudging the cut. Additionally, perceived size is more a function of diameter than it is stone depth. In inexpensive stone types like colorless Topaz, I say don't worry so much about saving weight, make the most attractive stone possible.

Keep the girdle in the range of 0.5 to 1.0 mm and it will look great. It will be easy to set in any style mounting, strong enough, and attractive. Make sure you polish it. A hand turned, slightly rounded girdle is much less prone to chipping that a flat faceted one because there are no sharp facet junctions to chip.

You have chosen a Portuguese style pavilion. So am I correct in assuming that you will also be cutting a Portuguese style crown? If so the crown angles are not as important as in a standard stacked main round brilliant cut. 45 degrees for you first tier (touching the girdle), should work out fine. More important will be crown height, the number of tiers you cut, and table size. Try to keep the table size somewhere between 53% and 60% of the stone diameter. Too small a table will result in a dark dead center of the stone, too big and you will start to see the girdle reflected in the stone. The number of tiers this takes will be determined by the starting angle, and the difference in angle between the tiers. This will angle difference will also determine the shape of each facet in a tier. Find the angle difference that has the facet shape matching what you have already done on the pavillion. Keep the shape the same for each tier. Cut the number of tiers it takes to get to your target table size. This of course will also set your overall crown height.

Following this cutting plan will get you to a well proportioned, brilliant, and attractive stone.

Let us know how it comes out.


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 Post subject: Re: How thick is too thick for a girdle?
PostPosted: Fri Aug 28, 2020 4:30 pm 
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azrugger wrote:
Thanks - the more I look at it the more awkward the giant girdle looks. I still want to preserve as much weight as possible though, so i wonder how feasible it is for my break facets dronbe at like 45d.

I know the critical angle of topaz is around 38d — so would that prevent light from entering via those facets in a meaningful way?


The majority of light entering the stone enters thru the table and returns back off the pavilion facets if not below the critical angle where if so, the light passes threw and doesn't return called windowing. Most will advise to stay 1.5 to 2 degrees above critical angle on pavilion mains.
Only a small amount light travels through small triangular crown girdle facets on round style designs but will reflect the light off of them giving them that twinkle or blinking as the stone is tilted. Portuguese style cuts have a lot of small facets and many tiers 4-5 on most and more on some. Portuguese style cuts are a design essentially to retain weight and mostly in the pavilion and the crowns are typically very shallow in comparison to the pavilion.

Based on the previous 1Bwanna1 post and I agree on his recommendations, I ran a set of angles through GemCad/GemRay for the RI of topaz, 64 index, colorless, a pavilion of 5 tiers mains @41.7, girdle facets @61, and a crown of 5 tiers, mains @28, girdle break facets at @35 gave the stone a 53% window, These gave the stone a very good performance of light return. Raising the 35 degree breaks start shrinking the window percentage, this is based on a change of 5 degrees between tiers. Like said with a @45 degree break facet, the elevation angles between tiers can be adjusted to compensate for the window percentages.

I have no idea what your pavilion angles are so do not take this as a recommendation to finish your stone at, but just some helpful info.

No matter what you do, it will look fine and your gaining experience. And you should be able to find a mounting, many 14mm mountings will work fine for a 13.7mm stone.

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 Post subject: Re: How thick is too thick for a girdle?
PostPosted: Mon Aug 31, 2020 2:14 am 
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Brought it down to 1mm for the girdle, and finished up the polish today.

Man, topaz is a bit of a bear to polish, isn’t it? I’m using a phenolic lap with Aluminum Oxide — I wonder if I’d be better served trying to polish on my tin lap in the future (realistically, I need to start purchasing a library of laps - so while phenolic May work great for beryl and quartz, it may be time to invest in some heavier duty polishers so I can break into sapphires and such)

Ended up, even after removing so much from the girdle - at 14.25ct. Now just gotta wait for my setting to arrive in the mail. Anyway, figured I’d share the results. Thanks so much for your help!


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 Post subject: Re: How thick is too thick for a girdle?
PostPosted: Mon Aug 31, 2020 10:42 am 
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That girdle looks bigger than 1mm to me. But no matter what it measures, it is way too thick for that stone.

In any case congratulations on your first Portuguese cut. They a fun to cut, and do great things for stones that are on the large size, and light to medium in color tone. As the light bounces around inside the stone off the many small facets of the Portuguese cut,
it lengthens the line the light travels. The amount of color light absorbed is proportional to the length of this line, so this darkens and evens the color. The Portuguese has lots of scintillation and sparkle, but is not as brilliant as other round facet patterns.

I always polish topaz with diamond. True for almost every stone except quartz these days.


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 Post subject: Re: How thick is too thick for a girdle?
PostPosted: Mon Aug 31, 2020 3:13 pm 
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Good job! I agree the girdle is way to big, but its your stone and you are new at this so what.
Maybe spend some time looking at the proportions of other stones online, maybe at your local jewelers have a visit and look at some of the stones already set if its to hard to judge in an online image on girdle widths. You have gained some experience, your learning the machine you have and any little quirks it may have, you should be proud. What's your next gem cutting adventure?

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 Post subject: Re: How thick is too thick for a girdle?
PostPosted: Mon Aug 31, 2020 4:10 pm 
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Well shoot about it being too big.

Oh well, this was primarily about how to cut and polish topaz, and identify some of the pitfalls such as with polishing. I’m not quite ready to upgrade all of my laps to different diamond grits (but eventually!)

I’m still not 100% confident in my ability to get a straight girdle, which is why I’ve been so cautious about going thin... but maybe on my next stone I’ll go for it. This one did turn out pretty straight, if not a bit too thick.

My next gem, dopped up, is going to be something a bit more relaxing that I don’t really have to learn on. So I’ve got a beryl that I bought in a lot off of Etsy, and cleaned away all the inclusions. It’s small, but it will be fun. Haven’t decided on a cut just yet.


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