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 Post subject: what does open C mean?
PostPosted: Thu Jan 29, 2015 8:40 am 
Been cutting over 2 years now, and cant find an answer to this probably basic question. What does open C mean? Image
Just throwing in a pic of aqua rough.


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 Post subject: Re: what does open C mean?
PostPosted: Thu Jan 29, 2015 9:10 am 
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I learned that lesson the hard way...I went to Tucson the first year after I started cutting and bought some beautifully rich green tourmaline that was flawless and a decent size.
I got home and cut a round brilliant out of the biggest pieces and ended up with a black stone that if help up to an intense light just right had flashes of that wonderful green I bought...but only when looking at the tiny tiny string of tables that connect up around the middle that most people call the girdle :D

crystals have a,b,and c axis in how they grow...the c axis in a crystal with a pencil like shape like tourmaline is when you look at the end of the pencil....if you see light at the end it considered "open c"....the degree to which light passes through to your eye varies as does the color in some to a strong degree.
different gemstones have this to different degrees and some not at all

If no light passes through an axis of a stone be careful selecting a cut that will eliminate it or by pass it altogether....gearloose had a interesting take on closed c tourmaline....it definitely sparkled and was VERY quick to cut!!.....I 'll se if I can find it for you.


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 Post subject: Re: what does open C mean?
PostPosted: Thu Jan 29, 2015 9:14 am 
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viewtopic.php?f=8&t=11479&hilit=closed+c+tourmaline+cutting+pattern&start=15


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 Post subject: Re: what does open C mean?
PostPosted: Thu Jan 29, 2015 9:45 am 
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if one would cut the table parallel to the c-axis, would the stone still appear very dark due to inner reflections of the dark component on the stone facets?


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 Post subject: Re: what does open C mean?
PostPosted: Thu Jan 29, 2015 10:08 am 
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Not sure what the max and minimum angles are but anytime the light crosses the plane of the c axis it is extinguished in completely closed c axis materials the only light you will see from stones with this problem will be light that enters and exits with out crossing the c axis....so there would be zero light return from a table on the c axis cut stone. Unless of course you turn the stone and look at the tiny tabes described in my first post here :D


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 Post subject: Re: what does open C mean?
PostPosted: Thu Jan 29, 2015 11:47 am 
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cascaillou wrote:
if one would cut the table parallel to the c-axis, would the stone still appear very dark due to inner reflections of the dark component on the stone facets?

There are special cuts for tourmaline to do this. It requires very steep ends on your stone to minimize the c axis's influence, makes it a bit of a nightmare to set and the stone will still be dark. This can salvage a sellable stone but it's probably not going to be at all how you hoped. They are sometimes also intentionally windowed, which can help though the downsides to that are obvious.

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 Post subject: Re: what does open C mean?
PostPosted: Thu Jan 29, 2015 12:07 pm 
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there are cuts that can salvage a closed c stone if it has the proper lenght to width ratio.....but I have never seen one that has the table cut on the c axis


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 Post subject: Re: what does open C mean?
PostPosted: Thu Jan 29, 2015 12:24 pm 
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Yeah, that would never work.

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 Post subject: Re: what does open C mean?
PostPosted: Thu Jan 29, 2015 1:31 pm 
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To cut a stone with a dark C axis, the pavilion facets on the C axis need to be steep,above 50 degrees or more, with higher desirable.

Here are some of Jeff Graham's design that work well with closed C :

(You can see the designs at http://www.jewelcutter.com/graham/designs/ without the detailed cutting info)

LONG SHORE MAN : here the P3 facets are cut at 68 degrees. Note that in this design, with a L/W = 1.25, the closed C is on the LONG side.

GENTLEMAN'S CHOICE : Here the P7 facets are on the closed C axis, and, again, they are on the long side of the stone (under C4) , with a L/W = 1.128 The angle used here is 52 degrees, the shallowest of any of these designs.

ELIZABETH'S FLASH : Here the P4 facets are on the closed C (under C2) and are cut at 69 degrees. In this design, the P4 facets are on the SHORT ends of this L/W = 1.200 design

SQUARE ZIGGIE (one of my favorite names for a design) : Here the P3 facets, cut at 72 degrees, are on the closed C in this square, L/W = 1, design.

LONG DIAMOND : Here the P4 facets, cut at 66 degrees, are on the closed C, at either end of a L/W ) = 2.0 long design. This is for long crystals. Also works for green tourmaline when the C axis has a bad color, for the same reasons.

SIMPLE CHECK : The P4 facets are on the closed C ends of this L/W = 1.336 design, under C2, and are cut at 69 degrees.

As you can see, the L/W can vary, and the closed C can be either on the ends or sides. Note, these closed C facets are cut ON the C axis, at 90 degrees to the A and B axis.

I have cut all these designs, usually with a closed C stone. A few years ago, a club member had bought a parcel of blue tourmaline that were a beautiful light blue on the A and B axis', but CLOSED on the C axis. He cut a 2mm thick slice off of the C axis on one stone and, even at that thinness it was dark. He had cut the stone and could not return the parcel, as he had also cut one in a SRB and it was black with some blue flash as it was tilted, and was going to use the remaining stone to amuse goldfish. I asked for the stones and did a trade. I cut the stone he had taken the slice from in Elizabeth's Flash. It was a beautiful blue tourmaline stone and he was surprised. We undid the trade and he gave me a couple stones in thanks. These designs really do work with closed C rough.

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 Post subject: Re: what does open C mean?
PostPosted: Thu Jan 29, 2015 2:01 pm 
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I should take a step back and say that my comment about them never quite measuring up is in reference to the more typical closed-C stones with a darker "AB" axis (actually just the A axes). Very interesting things can be done with the unusual case of a light AB and closed C, like this incredible Afghan closed-C green tourmaline cut in a Smith Bar (http://instagram.com/p/nh4rlzk5cf/)

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 Post subject: Re: what does open C mean?
PostPosted: Thu Jan 29, 2015 4:21 pm 
Wow, the matrix pair are, I dont know what to say! Farout comes to mind.
Thanks for the replies, I now know what closed c axis means.


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 Post subject: Re: what does open C mean?
PostPosted: Thu Feb 12, 2015 4:38 am 
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Ya, that matrix pair is looking pretty digital! Thanks a ton scarodactl.
Someday I'll cut a tourmaline, but learning this stuff is essential. Thanks to all the contributors. I'ma have to process this a little, and may have a couple to a few questions. It takes me a little time to process things sometimes.

HA HA, Jeff's "Patience 2" cut! 668 facet meetpoint cut!

You know, I'm not trying to change the subject here, but wow. I needed to see that "Patience 2" cut though.

I may be able to keep that goal in mind as I learn faceting.

There's gonna be stuff that I freestyle, fix mistakes, and roll with, and there's gonna be ultra as precise as possible patience projects.

If I can keep that cut in mind as goal, I believe I could be pretty happy in life.

I really do not play when it comes to accuracy in some things, and glad I don't.

I learned this type of mentality when I taught myself woodworking. Even on 2x4 and 4x4 post table building, etc, cubbyhole storage desks, etc ....

I'll spend 20 minutes setting the rail on the tablesaw!

Learning to mark the 2x4's sure taught me a lot too! What side of the mark? Kerf?Calibrating tape measures?Acid etch markings? How sharp is the pencil? How big is the tip on the pencil? Whats the shape of the tip on the carpenter pencil? How was my marking stroke? How far did the rule move when I marked it? How much pressure did I use?

Learning this somewhat useless stuff on 2x4/lag bolt projects kinda transfers over to everything else.

It really wasn't that long ago, I really knew nothing about building anything! Much clearer head now, and willing to save a little time for these patience side endeavors.


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 Post subject: Re: what does open C mean?
PostPosted: Thu Feb 12, 2015 4:55 am 
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Wow, yes. My faves of Jeff's are Elizabeth's flash, then the more traditional like-long diamond. I will have to try a Elizabeth's flash someday. Great use of the stone.


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