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 Post subject: Selecting a design based off of RI
PostPosted: Wed Jul 22, 2020 2:53 pm 
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So I bought this 43ct almandine garnet to practice on off of eBay (I know, I know, judge my choices here - but I'm not looking to ruin any quality stones while I learn).

Image

I know I need to take the shape of the stone into consideration, and figure out if there are any inclusions that I need to be aware of - but with that in mind, I want to understand how the RI of the mineral plays into the selection of the design. I've noticed that pretty much every design has an "angles for R.I." value listed. Am I guessing correctly that I look for an R.I. that closely matches Almandine? So ~1.8? Obviously, I don't have a refractometer so it's going to be based off of the reference, and I'm taking the ebay user's word that it's actually an almandine. But it was cheap so I'm not really concerned if I got "ripped off" for less than $20. I just want to optimize my process based on what I have.

Thanks!


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 Post subject: Re: Selecting a design based off of RI
PostPosted: Wed Jul 22, 2020 3:48 pm 
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Hi,

to me that Almandine lacks transparency and would not be good for faceting.

Have a look at Al Balmers suggestion
viewtopic.php?f=8&t=25770

You will find a lot of answers in the mentioned book.

https://gemologyproject.com/wiki/index. ... ng_Designs allows you to find designs by refractive index


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 Post subject: Re: Selecting a design based off of RI
PostPosted: Wed Jul 22, 2020 5:41 pm 
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RI is meaningless if the material is not transparent.


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 Post subject: Re: Selecting a design based off of RI
PostPosted: Wed Jul 22, 2020 5:47 pm 
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This is a bad picture - I see that now. There is transparency on the garnet, I will see if I can get a better image and re-post.

That said - just trying to learn here - if I had a RI of 1.8, would I look for a design that lists 1.8? How much variance can you tolerate from that in the design you pick?


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 Post subject: Re: Selecting a design based off of RI
PostPosted: Wed Jul 22, 2020 5:51 pm 
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Azrugger, Have you considered purchasing a synthetic boule (or several) of YAG?
RI=1.81
http://www.gemcutter.com/syn-ruf.htm


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 Post subject: Re: Selecting a design based off of RI
PostPosted: Wed Jul 22, 2020 5:55 pm 
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Barbra Voltaire, FGG wrote:
Azrugger, Have you considered purchasing a synthetic boule (or several) of YAG?
RI=1.81
http://www.gemcutter.com/syn-ruf.htm


Definitely not something I’ve considered, but will definitely now! That looks particularly affordable.

Still getting my legs on where to source things to cut in general. Thanks!

As far as an updated garnet photo - may be a while. If I hold it up to my iPhone light, I can see light coming through the edges - just not where it’s thick in the center. However, since I need my phone to take the picture, that kind of makes it difficult!


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 Post subject: Re: Selecting a design based off of RI
PostPosted: Wed Jul 22, 2020 6:01 pm 
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You know, I might keep that garnet as a crystal specimen and buy some various synthetic boules.

Did you also notice on the site:
Faceting Angles for Various Man-Made Materials


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 Post subject: Re: Selecting a design based off of RI
PostPosted: Wed Jul 22, 2020 7:18 pm 
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One of the things I see beginners, and some long time factors, focus on is the R.I. and critical angle. However, this has much less to do with what angles one chooses to cut at that many believe.

R.I. does set the critical angle of a material. The CA defines at what angle light hitting a surface is either reflected/refracted, or passes through. The RI is calculated by that angle when light traveling in air meets the material. So it is most important for when light hits the surface of the stone.

Once inside the stone Snell's Law takes over. This basically means that "the angle of incidence is equal to the angle of reflection". As long as this angle is above the CA the light will be reflected. So RI has very little impact as long as the angle is above the CA. It is a bit more complicated by secondary, tertiary, and more reflection paths. These are usually negatively affected by too high of angles in the pavilion.

Below is a link to a table of RIs and CAs for a variety of gem materials. You will notice that no matter what the RI and CA are, the recommended cutting angles don't change much. For the pavillion almost always between 40 to 44 degrees. Vargas seems to have the least variance. Crown angle have almost no effect on windows, but can affect the contrast, dispersion, and pattern of a cut.

http://www.gemcutter.com/angles.htm

So, as a general rule, unless your material has an extremely low RI if you keep you pavillion angles above 41 degrees, you will not be creating windows in your stones.

It is better to focus on other things like managing yield, color, inclusions, and workmanship issues like symmetry, and polish.


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 Post subject: Re: Selecting a design based off of RI
PostPosted: Thu Jul 23, 2020 3:27 am 
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That garnet is not faceting rough. It will cause you grief. Rather follow Barbra's suggestion to start cutting YAG, or even glass. Glass marbles or thick bottle punts make good practice material, and can produce interesting 'stones'.


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 Post subject: Re: Selecting a design based off of RI
PostPosted: Thu Jul 23, 2020 11:22 am 
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Duncan Miller wrote:
That garnet is not faceting rough. It will cause you grief. Rather follow Barbra's suggestion to start cutting YAG, or even glass. Glass marbles or thick bottle punts make good practice material, and can produce interesting 'stones'.


I plan to - I'm going to order some YAG today (as well as some Mt. St. Helens ash glass from that same site, since I've got an entire family from Oregon who lived through it, and that sounds like a really cool gift to give.

That said, what am I missing here? Aside from just getting a substandard gem out of the garnet, why would cutting it cause me grief? I want to learn how to cut something harder than quartz - and figure out my tools and processes in the meanwhiles. I don't think I would get that same experience cutting glass - as it will be soft like quartz.

Is there some reason I shouldn't tackle this while I wait for the other stuff I'm ordering to arrive?


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 Post subject: Re: Selecting a design based off of RI
PostPosted: Thu Jul 23, 2020 12:15 pm 
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I'm not a cutter, but my first thought is garnet is NOT harder than quartz.
Secondly the garnet xl is riddled with fractures which will likely shatter when you attempt cutting.

If you have your heart set on cutting that nightmare, go ahead. It's your time to waste.


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 Post subject: Re: Selecting a design based off of RI
PostPosted: Thu Jul 23, 2020 12:19 pm 
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Barbra Voltaire, FGG wrote:
I'm not a cutter, but my first thought is garnet is NOT harder than quartz.
Secondly the garnet xl is riddled with fractures which will likely shatter when you attempt cutting.

If you have your heart set on cutting that nightmare, go ahead. It's your time to waste.


Almandine is 7.5-8.5 depending on the specimen.

Amethyst is 7.

I know that you're supposed to use different polishing techniques for garnet as opposed to quartz because of the hardness - for example you have to use Aluminum Oxide rather than Cerium Oxide because the cerium won't touch it.

I didn't mean any offense, I'm just trying to see if there's some reason everyone seems to be suggesting glass to me instead of cutting this - I'm by no means an experienced cutter, so any experience is good at this point, right? I hear the criticism about the fractures - but it seems like a lot are surficial. I guess I'll see once I preform it.


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 Post subject: Re: Selecting a design based off of RI
PostPosted: Thu Jul 23, 2020 1:06 pm 
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Do what you want. I think we have done all we can for you.


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 Post subject: Re: Selecting a design based off of RI
PostPosted: Thu Jul 23, 2020 2:56 pm 
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azrugger wrote:
Almandine is 7.5-8.5 depending on the specimen.

This is incorrect in a few fairly dramatic ways. I know you were just going by what google says, and Google in turn pulled it off the wikipedia page (which actually says 7-7.5, but was likely vandalized when Google pulled it), but it's important to know that a lot of data gets exaggerated on various sites. Lots of times people selling particular soft gems will outright lie about hardness stats to confuse the issue. Better to listen when someone like Barbra (with a degree in geology and a long career of serious gemological work) offers you information.

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 Post subject: Re: Selecting a design based off of RI
PostPosted: Thu Jul 23, 2020 3:05 pm 
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Garnet is harder than quartz, a simple fact. However, quartz is tougher than garnet. Toughness is a different thing.

Despite garnet being harder, quartz is actually more difficult to cut and polish in my experience. It is also much dirtier to work with

With garnet, use diamond. It loves to be polished with diamond. Quartz, not so much.


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