jasita
Rough_Rock
- Joined
- Apr 29, 2004
- Messages
- 19
I sure do wish I knew what those gemology words meant. Maybe if we had an online discionary for the newbie. Just a thought Richard. I'm really NOT clear on the words. Maybe its just me, but I am lonely at the top..lol
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On 11/21/2003 10:59:59 PM Richard Sherwood wrote:
As you can see from the attached diagram, the visual optics technique is based on using the pavilion facet angles of the test stone as prisms. You hold the stone’s table facet right up close to your eye until you can see through the stone. Usually the stone will be about to touch your eyelids. You then view a light source through the stone, preferably a small one (penlight, fiber optic), although you can use anything from the moon to a candle flame (which makes for a romantic setting).
The light enters through the pavilion facets, is spread into the rainbow spectrum (dispersion), bounces off the table and can be seen on the opposite pavilion facets as a “mini-spectrum”. In round stones you'll see multiple mini-spectrums goind around in a circle(s) according to the pavilion facet pattern arrangement.
On low refractive index stones you might observe 3 circular patterns, while a medium refractive index stone might exhibit 1 or 2, and a high refractive index stone only exhibiting the pattern if you tilt or lower the stone in relation to your eye. Very high refractive index stones (over 2.2) will not have circular patterns but rather “fields” of primary & secondary images which appear like snowflakes in and around your field of vision.
Several things to note about the mini-spectrum. The length of it will indicate the dispersiveness of the gem, how much it spreads the spectrum, resulting in “fire”. Leaded glass, for example, is highly dispersive and will have a long spectrum. Quartz has a low dispersion, with a short spectrum. This quality in itself is a helpful diagnostic.
Secondly, you will be able to tell if a gem is doubly refractive or singly refractive by whether or not the spectrum is “doubled”.
Thirdly, you will be able to guage the birefringence (how far the 2 rays are split) of the gem by how far apart the doubled images appear. Rotating a Polaroid filter between the stone and the light source further enhances this phenomena. A gem with high birefringence will show the two images completely separated, while a lower birefringence gem will show the two images touching, or even overlapping.
Fourth, This diagnostic feature is augmented by a B
ratio table the author has compounded, which shows the ratio between the separation of the two spectrums (birefringence), divided by the length of the spectrum (dispersion).
Fifth, you can tell by how far away from the center of the stone that the pattern (referred to as the retina pattern) is reflected whether or not the stone has a low, medium or high refractive index. In fact, you can get results with the accuracy of a traditional refractometer if you combine this technique with a table top refractive index gauge which the author sells or which you can make yourself.
Sixth, you will often be able to spot major absorption patterns evident within the spectra. Not the fine stuff, you’ll need a spectroscope for that. But you can easily spot the difference between the spectrum of a ruby versus a garnet, for example. Both have clearly evident spectra, totally different from each other. This comes in handy for mounted stones which can’t be placed flush on a refractometer, among other things.
It also comes in handy when you’re out in the “field” having to make snap judgements about a particular piece you’re viewing.
Seventh, this technique can be a life saver when you become familiar with the “retina patterns” of diamond vs cubic zirconia vs moissanite. Each has a distinctive look, with the double refraction of moissanite providing an additional diagnostic.
Eighth, the symmetry and polish of gems can be somewhat evaluated with this technique (symmetry on gems with a refractive index of less than 2.2). The arrangement and spacing of the retina pattern will give you a clue as to the symmetry of the stone, while the crispness or blurriness of the image will give you a clue as to it’s polish.
Some examples of retina patterns and spectral images follow:
(Notice the doubled images of the zircon mini-spectrum)----------------[/quote]
blockquote style="padding-bottom:0;marginTop:0;marginBottom:0;">----------------
On 11/21/2003 10:59:59 PM Richard Sherwood wrote:
As you can see from the attached diagram, the visual optics technique is based on using the pavilion facet angles of the test stone as prisms. You hold the stone’s table facet right up close to your eye until you can see through the stone. Usually the stone will be about to touch your eyelids. You then view a light source through the stone, preferably a small one (penlight, fiber optic), although you can use anything from the moon to a candle flame (which makes for a romantic setting).
The light enters through the pavilion facets, is spread into the rainbow spectrum (dispersion), bounces off the table and can be seen on the opposite pavilion facets as a “mini-spectrum”. In round stones you'll see multiple mini-spectrums goind around in a circle(s) according to the pavilion facet pattern arrangement.
On low refractive index stones you might observe 3 circular patterns, while a medium refractive index stone might exhibit 1 or 2, and a high refractive index stone only exhibiting the pattern if you tilt or lower the stone in relation to your eye. Very high refractive index stones (over 2.2) will not have circular patterns but rather “fields” of primary & secondary images which appear like snowflakes in and around your field of vision.
Several things to note about the mini-spectrum. The length of it will indicate the dispersiveness of the gem, how much it spreads the spectrum, resulting in “fire”. Leaded glass, for example, is highly dispersive and will have a long spectrum. Quartz has a low dispersion, with a short spectrum. This quality in itself is a helpful diagnostic.
Secondly, you will be able to tell if a gem is doubly refractive or singly refractive by whether or not the spectrum is “doubled”.
Thirdly, you will be able to guage the birefringence (how far the 2 rays are split) of the gem by how far apart the doubled images appear. Rotating a Polaroid filter between the stone and the light source further enhances this phenomena. A gem with high birefringence will show the two images completely separated, while a lower birefringence gem will show the two images touching, or even overlapping.
Fourth, This diagnostic feature is augmented by a B
ratio table the author has compounded, which shows the ratio between the separation of the two spectrums (birefringence), divided by the length of the spectrum (dispersion). Fifth, you can tell by how far away from the center of the stone that the pattern (referred to as the retina pattern) is reflected whether or not the stone has a low, medium or high refractive index. In fact, you can get results with the accuracy of a traditional refractometer if you combine this technique with a table top refractive index gauge which the author sells or which you can make yourself.
Sixth, you will often be able to spot major absorption patterns evident within the spectra. Not the fine stuff, you’ll need a spectroscope for that. But you can easily spot the difference between the spectrum of a ruby versus a garnet, for example. Both have clearly evident spectra, totally different from each other. This comes in handy for mounted stones which can’t be placed flush on a refractometer, among other things.
It also comes in handy when you’re out in the “field” having to make snap judgements about a particular piece you’re viewing.
Seventh, this technique can be a life saver when you become familiar with the “retina patterns” of diamond vs cubic zirconia vs moissanite. Each has a distinctive look, with the double refraction of moissanite providing an additional diagnostic.
Eighth, the symmetry and polish of gems can be somewhat evaluated with this technique (symmetry on gems with a refractive index of less than 2.2). The arrangement and spacing of the retina pattern will give you a clue as to the symmetry of the stone, while the crispness or blurriness of the image will give you a clue as to it’s polish.
Some examples of retina patterns and spectral images follow:
(Notice the doubled images of the zircon mini-spectrum)----------------[/quote]
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