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So I had another question about star sapphires.
Why do they have an unpolished but shaped backside when other cabs are flat on the bottom?
If the sole purpose was to maximize carat weight then why angle the backside in below the girdle? Why not just leave all the material that is below the girdle in a straight line? I feel like there is a physical optics explaination somewhere.
I also found this helpful article from ed at wildfish gems on evaluating star gems online:
Gemstone phenomena, especially asterism, are a tough call to judge on photo. However, no normal jeweler will be able to show you a fine natural star sapphire, let alone a selection to choose from. Good stars are rare even beyond the normal gemstone rarity. Unless you live in a metropolis or travel to Tucson or Basel, the internet is the only place to compare and buy such gems.
Looking at images on the web, stars seem to be quite an ugly bunch. Rarely do they show nice colors, often they are zoned, patchy, heavily included, silky, egg-shaped and at times the asterism is hardly visible at all.
And of course you will find many "perfect", "fully colored", giant star sapphires or rubies for a few dollars. These are synthetic or surface diffused or lead-glass filled gems which are mostly worth just as much as they cost.
There is nothing wrong with twenty carat Linde star for fifty dollar, but be wary of those sellers trying to offer them as real.
So, are they are either ugly or faked? No, don't be discouraged. Real, natural stars are mind-shaking and heart-breaking. Many star skeptics have become sworn star fans after their first encounter with fine quality.
Here is what to look for when selecting a star sapphire or ruby online:
The value of any star gem depends strongly on the quality of its asterism, which is defined by (no order):
Sharpness
Symmetry & linearity
Completeness (6 rays mostly)
Travel (smoothness of movement)
Position
Lucidity & Depth
The relative importance of these criteria are questions of personal taste, culture and fashion. Most collectors would perhaps trade in some off-centeredness for good movement, or overlook a meandering leg while frowning at a missing one.
We feel lucidity, travel, position and completeness may be most important and price relevant.
Only then, with decreasing relevance, come:
Color
Clarity (inclusions)
Shape
Finish (top and bottom)
Asterism and color together easily make up 80% of the value of a star (sapphire, ruby or any other variety).
With ten dimensions (as compared to the old 4 Cs) stars are a quite demanding topic. But they are rewarding, too.
Let us tackle each issue separately from the web's point of view:
1. Sharpness is easily shown on the web. There is an ideal distance between the spotlight placed on a gem and the gem bringing out the sharpest star. You may assume that the photo captures this point. (unless stated otherwise in the grading report) In general, a camera shows stars not as sharp as the eye perceives them. Hence, when you see a sharp star on photo you can safely conclude that the star is actually very good in person. If you can just make out the star on the photo, it will be fleeting and in need of a strong single spotlight. Please be advised that one may well find faked (photo-shopped) sharp stars on Ebay or elsewhere. As always, if it is too good to be true... and never buy without 3rd party certificate.
2. Symmetry and linearity too, are easy: What you see is what you get.
3. Completeness: No problem either. Count the legs. If the gem on the photo misses one leg it is likely missing for real (unless the grading says otherwise.)
4. Travel, the ability and smoothness of the star traveling over the stone, is not really possible to show on the web (not yet at least). Until online video technology has advanced further, you will have to rely on the seller's description and check later in person. Ideally, the star follows a light source smoothly, not jerking and jumping, while staying intact and sharp. Don't expect perfection though: Every star has a weak area or two. Value reducing characteristics would be a loss of completeness or even a sudden disappearance of the star during movement.
5. Position too, is a tricky business on the internet. In order to show the star on a photo one must bring it close to the center. Thus, on a photo, the star will usually be centered. But that does not mean it actually is. Even a 100% centered star would need camera and spotlight in the same position (which is next to impossible). Hence, position is always a criterion that one must question further even if the photo looks great. Sometimes you may see the light coming obviously from the side while the star sits centered, meaning that, when the light is moved up, the star will probably shift off-center. However, this is not 100% conclusive. Mostly you will need to rely on the seller's evaluation. Some online vendors will try to put a number to off-centeredness: "45 degree off-center" for example indicates that when the spotlight is positioned straight above, the center of the star sits half way down the stone. Ten degree would mean the star is just slightly off center and more than 60 degree would send it nearly over the edge.
6. Lucidity & Depth are connected. In one extreme we will have completely opaque material with the star sitting on the surface. Opaqueness is easy to spot on photos. It looks like a solid piece of material with the rays fixed or painted onto the surface. At the other end of the spectrum we may have highly transparent quality with the rays reaching into the body. One can, however, not expect 100% transparency because needles are necessary to break the light and show the star. If the needles are very fine and yet the star is clearly visible, then ... There is magic: The rays sway like silver curtains inside this most dense material.Most stars however are rather opaque characters and transparency is highly priced.
The remaining four points are also relevant in normal gems but some additions are helpful:
7. Color: Strong colors are extremely rare in stars. Even the best star will have a silky, silvery sheen to it which clouds its saturation. This is unavoidable. A star needs needles. Most stars come in grayish, foggy mild colors. Exceptions exist but they are very costly and yet can not rival the intenseness of a 100% transparent gem. Novices will be disappointed when expecting to find a neon red ruby or a hot pink sapphire with perfect asterism. One has to appreciate the silkiness as part of the phenomena, and see the color as an add-on. Even grayish white stars have good value if their asterism is of high quality. The main problem, however, when judging colors on photo springs from the fact that any spotlight, needed to show the star, affects the character of the color. There is no cure to this. If the spotlight alters the color too much we often add a "no-star" image to show the gem's "2nd" color. This means one image to show the star, and one more to show the color (without a spotlight) under mixed and diffused light conditions. However, this is not fully satisfying since star quality and color saturation interact. Again you have to read the sellers description carefully and trust his judgment until you can examine the stone yourself. There is a trade-off between a weak star and a full color: The finer the needles, the better the color potential, but the weaker the star. Hence, the worst of all possible stars will be valued close to a cabochon of similar quality. Thinking of ruby, we would, at some point, discard the asterism and cut the cabochon into a facetted gem with that sought-after silky ruby sheen.
8. Clarity: Stars are by nature often more included than normal facetted gemstones (beyond their needle structure). This may be connected to their geological origin as semi-transparent material. However, inclusions should not dominate the overall appearance or hinder the rays from traveling. Stars also often display stronger color zones. Here too, one has to be tolerant. As long as the star runs unhindered through those color zones you ar
Why do they have an unpolished but shaped backside when other cabs are flat on the bottom?
If the sole purpose was to maximize carat weight then why angle the backside in below the girdle? Why not just leave all the material that is below the girdle in a straight line? I feel like there is a physical optics explaination somewhere.
I also found this helpful article from ed at wildfish gems on evaluating star gems online:
Gemstone phenomena, especially asterism, are a tough call to judge on photo. However, no normal jeweler will be able to show you a fine natural star sapphire, let alone a selection to choose from. Good stars are rare even beyond the normal gemstone rarity. Unless you live in a metropolis or travel to Tucson or Basel, the internet is the only place to compare and buy such gems.
Looking at images on the web, stars seem to be quite an ugly bunch. Rarely do they show nice colors, often they are zoned, patchy, heavily included, silky, egg-shaped and at times the asterism is hardly visible at all.
And of course you will find many "perfect", "fully colored", giant star sapphires or rubies for a few dollars. These are synthetic or surface diffused or lead-glass filled gems which are mostly worth just as much as they cost.
There is nothing wrong with twenty carat Linde star for fifty dollar, but be wary of those sellers trying to offer them as real.
So, are they are either ugly or faked? No, don't be discouraged. Real, natural stars are mind-shaking and heart-breaking. Many star skeptics have become sworn star fans after their first encounter with fine quality.
Here is what to look for when selecting a star sapphire or ruby online:
The value of any star gem depends strongly on the quality of its asterism, which is defined by (no order):
Sharpness
Symmetry & linearity
Completeness (6 rays mostly)
Travel (smoothness of movement)
Position
Lucidity & Depth
The relative importance of these criteria are questions of personal taste, culture and fashion. Most collectors would perhaps trade in some off-centeredness for good movement, or overlook a meandering leg while frowning at a missing one.
We feel lucidity, travel, position and completeness may be most important and price relevant.
Only then, with decreasing relevance, come:
Color
Clarity (inclusions)
Shape
Finish (top and bottom)
Asterism and color together easily make up 80% of the value of a star (sapphire, ruby or any other variety).
With ten dimensions (as compared to the old 4 Cs) stars are a quite demanding topic. But they are rewarding, too.
Let us tackle each issue separately from the web's point of view:
1. Sharpness is easily shown on the web. There is an ideal distance between the spotlight placed on a gem and the gem bringing out the sharpest star. You may assume that the photo captures this point. (unless stated otherwise in the grading report) In general, a camera shows stars not as sharp as the eye perceives them. Hence, when you see a sharp star on photo you can safely conclude that the star is actually very good in person. If you can just make out the star on the photo, it will be fleeting and in need of a strong single spotlight. Please be advised that one may well find faked (photo-shopped) sharp stars on Ebay or elsewhere. As always, if it is too good to be true... and never buy without 3rd party certificate.
2. Symmetry and linearity too, are easy: What you see is what you get.
3. Completeness: No problem either. Count the legs. If the gem on the photo misses one leg it is likely missing for real (unless the grading says otherwise.)
4. Travel, the ability and smoothness of the star traveling over the stone, is not really possible to show on the web (not yet at least). Until online video technology has advanced further, you will have to rely on the seller's description and check later in person. Ideally, the star follows a light source smoothly, not jerking and jumping, while staying intact and sharp. Don't expect perfection though: Every star has a weak area or two. Value reducing characteristics would be a loss of completeness or even a sudden disappearance of the star during movement.
5. Position too, is a tricky business on the internet. In order to show the star on a photo one must bring it close to the center. Thus, on a photo, the star will usually be centered. But that does not mean it actually is. Even a 100% centered star would need camera and spotlight in the same position (which is next to impossible). Hence, position is always a criterion that one must question further even if the photo looks great. Sometimes you may see the light coming obviously from the side while the star sits centered, meaning that, when the light is moved up, the star will probably shift off-center. However, this is not 100% conclusive. Mostly you will need to rely on the seller's evaluation. Some online vendors will try to put a number to off-centeredness: "45 degree off-center" for example indicates that when the spotlight is positioned straight above, the center of the star sits half way down the stone. Ten degree would mean the star is just slightly off center and more than 60 degree would send it nearly over the edge.
6. Lucidity & Depth are connected. In one extreme we will have completely opaque material with the star sitting on the surface. Opaqueness is easy to spot on photos. It looks like a solid piece of material with the rays fixed or painted onto the surface. At the other end of the spectrum we may have highly transparent quality with the rays reaching into the body. One can, however, not expect 100% transparency because needles are necessary to break the light and show the star. If the needles are very fine and yet the star is clearly visible, then ... There is magic: The rays sway like silver curtains inside this most dense material.Most stars however are rather opaque characters and transparency is highly priced.
The remaining four points are also relevant in normal gems but some additions are helpful:
7. Color: Strong colors are extremely rare in stars. Even the best star will have a silky, silvery sheen to it which clouds its saturation. This is unavoidable. A star needs needles. Most stars come in grayish, foggy mild colors. Exceptions exist but they are very costly and yet can not rival the intenseness of a 100% transparent gem. Novices will be disappointed when expecting to find a neon red ruby or a hot pink sapphire with perfect asterism. One has to appreciate the silkiness as part of the phenomena, and see the color as an add-on. Even grayish white stars have good value if their asterism is of high quality. The main problem, however, when judging colors on photo springs from the fact that any spotlight, needed to show the star, affects the character of the color. There is no cure to this. If the spotlight alters the color too much we often add a "no-star" image to show the gem's "2nd" color. This means one image to show the star, and one more to show the color (without a spotlight) under mixed and diffused light conditions. However, this is not fully satisfying since star quality and color saturation interact. Again you have to read the sellers description carefully and trust his judgment until you can examine the stone yourself. There is a trade-off between a weak star and a full color: The finer the needles, the better the color potential, but the weaker the star. Hence, the worst of all possible stars will be valued close to a cabochon of similar quality. Thinking of ruby, we would, at some point, discard the asterism and cut the cabochon into a facetted gem with that sought-after silky ruby sheen.
8. Clarity: Stars are by nature often more included than normal facetted gemstones (beyond their needle structure). This may be connected to their geological origin as semi-transparent material. However, inclusions should not dominate the overall appearance or hinder the rays from traveling. Stars also often display stronger color zones. Here too, one has to be tolerant. As long as the star runs unhindered through those color zones you ar
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