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 Post subject: A little help, please?
PostPosted: Wed Oct 10, 2012 6:29 am 
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It's funny, the further I go down the rabbit hole, the more amazed I get; I understand more and more, but realize that what I really know amounts to nothing.

I'm trying to understand the world of rough gemstones. I get that the color of a cut gemstone is largely based off of the reflected light from the facets from the back side of the stone, some of it gets absorbed by whatever that particular variety of stone uses as a source of color (inclusions, color centers, impurities). I've read the blog post which helps you choose rough, and a large portion of it makes sense. What confuses me is when you get a very clean piece of rough, do the same rules apply? The idea of the white paper test was to look at the reflected light on the paper behind, and the idea is that's the color you can expect from the gemstone.

Here's a picture of a rough Topaz I just bought, it's 309.7 ct, it's transparency is fairly obvious. No spotlight in the background, just the lights from my apartment. From the previous set of rules, the color is either the color on the flat surface, or practically none as viewed through the near glass stone:


Certain rough starts out somewhat faceted to start, if I recall correctly, topaz is one such stone. Lying the stone on my pant leg, and snapping a shot showing its natural facets, reveals a yellowish hue:


Sitting the stone down on its flat base, gives an odd red color to the included faces:


In these images there's two light sources: one behind me and one in front of me. The one with the yellow color has the light mostly above it to best facilitate the reflection off of the back faces.

When light isn't bouncing off inside of the stone, looking through it, it seems almost colorless, can someone assist and help explain what's going on? I'm curious as to how to identify what kind of color the stone would yield, if any. I'm quite perplexed with this one because most of what I've bought up until now has been glassy, seriously color zoned, and/or just too included to do anything with. I did admittedly pay more for this piece though.

Any insight is more than welcome, as I haven't the slightest clue of how to evaluate this rough for color.

Edit:
It could just be me (and it probably is), but it oddly appears more orange/red in incandescent light. I give up on trying to figure out what color this will yield :?


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 Post subject: Re: A little help, please?
PostPosted: Wed Oct 10, 2012 12:12 pm 
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Some topazes are pleochroic, appearing yellow from the sides of the crystal, and red down the long axis. This is particularly pronounced in Guererro Topaz (Mexico)
(OLD picture from camcorder and Snappy sreen grabber.)
Image
I even have a blue topaz, of good London color, that is water-white transparant when viewed from the side.


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 Post subject: Re: A little help, please?
PostPosted: Wed Oct 10, 2012 3:36 pm 
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I had read that topaz was dichroic but hadn't seen it so pronounced. It might explain why I've seen samples of the faceted stone with a different colored rim around the edges. It looks a lot ... different in an uncut stone.

Is the rough pictured representative of the irradiated junk that fades? If so, how would I test for that: does it require direct sunlight (from say a window that's facing east or west) or indirect sunlight (from a window facing north or south, receiving sunlight reflected off of the atmosphere.)

Edit:
So from what you can tell, would you say the material is from Mexico?


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 Post subject: Re: A little help, please?
PostPosted: Wed Oct 10, 2012 4:04 pm 
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To me, it looks like it might be of the fading type. Note that not all fading topaz is brown or brown-orange; there is even some really fantastic red-orange stuff can fade to colorless in sunlight (which I have learned much to my lasting regret). Before investing in cutting, I would set it out in the sun for a while to see what it will do.

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 Post subject: Re: A little help, please?
PostPosted: Wed Oct 10, 2012 4:10 pm 
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How long, and ... in direct sunlight? Inside a car, or would the heat be a problem?


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 Post subject: Re: A little help, please?
PostPosted: Wed Oct 10, 2012 4:26 pm 
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I have tested it personally on a dark, cola-colored (and obviously irradiated) crystal, in direct sunlight, which had faded noticibly within a couple of hours. A friend of mine tested the red-orange material under a broad-spectrum halogen light, and it faded to colorless in a day. While that will heat them some, I don't think it would be enough to cause independent change to the stone's color.

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 Post subject: Re: A little help, please?
PostPosted: Wed Oct 10, 2012 4:30 pm 
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All transparent, colored gem materials will absorb different colors of the light them at differing rates as the light travels through the stone. Some colors are absorbed right away and other colors require a longer path length to be absorbed an equal amount. In addition to this your eyes will see more intense light as being lighter in hue and tone. The end result of all this is that the cut stone will have many different colors, (usually variations of it's body color, but sometimes not).

Remember that a light bulb is NOT the only source of light being reflected by the gem. The walls, table, paper and everything else in the room is providing a source of light which may be strong enough to be transmitted through and reflected by the gem. The difference between that part of the gem which is reflecting a satin wall paint and that which is directly reflecting the light from a light bulb is very dramatic and the colors which you are seeing in that rough show you pretty clearly the same colors that will be seen in the finished cut stone. It will vary depending on how the stone is tilted, what the light source is, and on and on, but in a well cut stone the lighter orange reflected color that you're seeing is about what you'll get, (of course the finished stone will most likely be smaller, have shorter light paths and be brighter and appear lighter in hue). The only way to know for sure is to cut the stone and see!

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 Post subject: Re: A little help, please?
PostPosted: Wed Oct 10, 2012 4:40 pm 
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What are some clear indicators of an irradiated stone, are there any physical characteristics that might give it away?


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 Post subject: Re: A little help, please?
PostPosted: Wed Oct 10, 2012 4:45 pm 
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(as an elaboration of what michael_e said, look at this picture of pumpkin seed oil. It is famous for being green in thin layers and dark red in thick layers.)
Image

I did some further reading, and it appears there is at least some subset of topaz which is sensitive to heat (even heat of cutting) but not to sunlight. I'm not sure what role that would play in your tests, since there was no temperature threshold given in the article, but yeah.


I'm no expert, but really wonky color zoning does appear to be an indicator of irradiation. I've seen, for instance, some with an x-shaped brown zone criss-crossing an otherwise clear/colorless crystal, which I have trouble believing was the result of natural chemical zoning in the crystal.

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 Post subject: Re: A little help, please?
PostPosted: Wed Oct 10, 2012 4:58 pm 
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Quote:
Is the rough pictured representative of the irradiated junk that fades?

No, this is the natural junk that fades.
Quote:
If so, how would I test for that: does it require direct sunlight (from say a window that's facing east or west) or indirect sunlight (from a window facing north or south, receiving sunlight reflected off of the atmosphere.)


I cannot find the thread but I did a posting showing deliberate fading of topaz by UV. I used a 254 nm germicidal UV lamp. Thus, I could take the pieces that did not fade from the rough parcel and be able to cut them with some confidence. Direct sunlight will work. In a window may not, as most window glasses block UV.


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 Post subject: Re: A little help, please?
PostPosted: Wed Oct 10, 2012 5:09 pm 
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How do you differentiate natural color zoning from unusual color zoning?

Is a striped, layered set of zones common for a stone or indicative of irradiation? Layers only visible from extreme angles when you're looking for them?


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 Post subject: Re: A little help, please?
PostPosted: Wed Oct 10, 2012 5:47 pm 
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Striped, layered zoning only visible from the sides is a feature that I have seen in irradiated topaz. I can't say it doesn't occur in natural topaz, though.

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 Post subject: Re: A little help, please?
PostPosted: Wed Oct 10, 2012 6:23 pm 
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So, I'll set it in the sun for a few hours. If it's stable it might just be worth cutting.


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 Post subject: Re: A little help, please?
PostPosted: Wed Oct 10, 2012 6:42 pm 
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I'd give it a week in direct sun. If it does not fade by then, it is probably safe to cut it.
It's quite awful to set one in gold, sell it, then see it two weeks later having become a pale champaign color... Hence the instant enthusiasm for pre-testing them.


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 Post subject: Re: A little help, please?
PostPosted: Wed Oct 17, 2012 1:18 am 
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Are there stones with one stable and one unstable color?

I decided to place it in my car (since I don't have a better place) and it got quite hot due to the sun. The red seems to have faded a bit, but I don't know if it's the sunlight or the heat, since it was untouchably hot.

Insight?


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