[NOTE: This is an archive of a Facebook post with a great discussion about the fluorescence of rubies] "The chromium present in ruby causes a natural red fluorescence which is stimulated by both visible and invisible (ultraviolet) light. However, when a ruby contains a significant amount of iron in addition to the chromium, the fluorescence is reduced. Rubies naturally fluoresce stronger to long wave UV than short wave. Stones with the strongest fluorescence are often a pinkish red color in daylight."
Bruce White Would love to see some examples of madagascar rubies brightly fluorescing in LW. Looking on Google images now, and do see a few, but would like to see more.
Bruce White Here's more evidence: http://www.lotusgemology.com/index.php/library/articles/322-blood-red-rubies-from-madagascar-lotus-gemology
Bruce White Looks like Madagascar is medium to strong fluorescence.
Bruce White Still, if anyone's got pics please post them.
Richard David Armstrong Fine quality Burmese rubies have the strongest fluorescence of natural rubies. However flame fusion synthetic rubies have a very strong fluorescence. I use a synthetic ruby to calibrate my spectrometer, because of the strong fluorescent (emission) line at 693nm which is seen without the use of a UV light source. Blue light will make some rubies fluoresce. Sunlight gives fine quality Burmese rubies a natural glow from the fluorescence.
Don Newsome As Wim pointed out ruby (corundum) does not always fluoresce. And when they do there can be a big difference in the luminance (brightness) of the red fluorescence. In fact corundum does not always fluoresce red, I have some that fluoresce orange, and I have seen one that fluoresced yellow. But the orange and yellow fluorescence is very rare.
Bruce White Don, under what conditions does ruby not fluoresce, and can you tell by the color?
Don Newsome Bruce White I do not know. And no you cannot tell by the daylight color (at least I cannot). I expect that the Convoy S2+ 365 will tell us a lot when it comes to corundum fluorescing.
Richard David Armstrong To answer the question. No, not all rubies are fluorescent. If they have a lot of iron in them they may not fluoresce as the iron supresses the fluorescent response. Those rubies usually come from a magmatic (igneous) environment. According to the GIA Lab Manual fluorescence of rubies is inert to strong.
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Bruce White Can you tell by color?
Richard David Armstrong Not sure of your question, but rubies that do fluoresce will always be red. I have used an LED light with a blue filter on rubies. Some rubies (synthetic and Burma) will appear pink to red while many others (Thai for example) will appear gray. A 405nm laser will really light up a lot of rubies.
Mark ColeGroup Admin Have never seen a ruby that did not light up under powerful excitation (laser, high power 365nm LED) but my sampling size is probably not as wide as others. White light color never gave me a hint about the intensity of the fluorescence
Charles Gould If corundum is red, it is ruby. All other different daylight colored corundums are sapphires.
Bruce White Charles Gould, right, but if a ruby is slightly brown with a bit more iron, then you could see that and know it's less likely to fluoresce?
Charles Gould Bruce White Probably, but just test the specimens with a light. I have noticed variations in the intensity of response of specimens from the same locality.
Bruce White Charles Gould, not possible when you're looking at an ebay auction with no fluorescent pics!
Charles Gould Bruce White ask the seller! What locality? If it looks red, it should be good. Anything else will not be exciting.
Richard David Armstrong From my experience I think that iron tends to give a purple component to ruby also reduces the saturation of the color. Some of the best fluorescence I have seen is with pink sapphire. However there is the question of when does corundum stop being rubyand becomes pink sapphire. Even my instructors when I asked this question did not have an answer. Best answer I have heard from anyone is "It depends on whether you are the buyer or the seller". Also too much iron can change the color to a purple sapphire.
Bruce White I just bought this. Do you think it'll fluoresce brightly?
Bruce White Luckily it's cool enough that even if it doesn't, I'll still keep it as a vis display piece.
Richard David Armstrong Bruce White I wouldn't call that ruby, too purple. Probably will not fluoresce. But try it and see, I could be wrong. That's what makes this hobby interesting specimens can sometimes surprise you.
Bruce White Madagascar
(Added the below pic after the rock was received - it did indeed fluoresce, very nicely.)
Richard David Armstrong I was just reading the section on luminescence in Ruby and Sapphire a Gemologists Guide, by Richard Hughes. He has a good discussion, a bit technical, on the luminescence of chromium in ruby. Too long to go into but one thing i got out of it is that the excitation wavelengths correspond to the absorption bands of the ruby spectrum, specifically 560 nm (green), 410 nm and 255 nm. Also there are 2 weak bands at sbout 475 nm. The fluorescence is at the 695 absorption band which is usually seen as an emission band in the visible spectrum. The strongest excitation is from the blue bands (if I understand correctly what he is saying) and the efficiency is about 70 to 80 percent.
Richard David Armstrong He also said the the chromium fluorescence process has been extensively studied by physicists because of the use of synthetic ruby in lasers. As I recall synthetic ruby was one of the first materials use as a laser. By increasing the Cr to about 0.50 percent it will also fluoresce in the infra-red.
Mark ColeGroup Admin Interesting coincidental post on mindat by Owen Lewis in answer to a post about identifying minerals based on fluorescence: "Sorry Alyson but I think you are chasing a moonbeam (or perhaps a rainbow might be more appropriate). UV fluorescence in minerals is frequently caused not by the mineral itself but by some trace impurity trapped in the crystal lattice or else in trace level substitution for one of the essential elements for the minerals make-up.
A good simple example is ruby, aka red corundum, aka red Al2O3 . In its pure state, Al2O3 is colourless and does not fluoresce. However, if close to 1% of the Al atoms are substituted by Cr, such a specimen will have a strong red colour and also UV fluoresce a strong red. Both the colouration and the fluorescence are related to the presence in the host mineral of the right chromophore element and in the right quantity. Drop the Cr substitution level below 1% and , as it reduces, the red fades through ever-weakening shades of pink until, eventually an observing eye can no longer detect any red at all. Go the other way and raise the Cr content above 1% and the attractiveness of the red reduces, until, at around 4% substitution, all red is gone and one is left with an unattractive brown specimen.
Change the chromophore to Fe+Ti in the correct ratio and with the correct ion charge states and you will have an attractive blue corundum stone (var. sapphire) that does not fluoresce at all. Change the chromophore to Fe only and the corundum will be either yellow or green, dependent on the ionic charge of the Fe.
It gets worse. Fe is the most common of a number of elements that, if present in corundum along with Cr, will act to prevent ('quench' is the term used) the UV fluoresence from a neighbouring Cr-substituted corundum molecule in the crystal lattice. Given the wide-spread presence of iron oxides in the Earth's crust, it is a commonplace to find rubies that only fluoresce weakly or even do not fluoresce at all, despite the correct-level presence of Cr that produces the correct red colouration. These have a less attractive red because of the loss of visual pizzazz given by the strong UV fluorescence that can be present in daylight.
- There is no way that fluoresence can ID correctly all samples of corundum (or anything else).
- Nor is fluorescence a reliable test (stand-alone) even for var. ruby, as the common presence in the lattice of a quenching agent (e.g. Fe) makes a test for UV fluorescence generally unreliable.
- Even where conditions are perfect and a ruby (e.g; from Mogok) fluoresces to look like a hot coal, such a result, stand-alone can't distinguish a Mogok ruby from a Mogok red spine, nor (with a bit more of a stretch) from unquenched Mogok chrysoberyl var. alexandrite.
Before spending time on developing a software driven colour grading system, you might want to:
- Study closely the fifteen causes of colour, of which the phenomenon of fluorescence is one.
- Review carefully the many colour grading systems already out there and in serious use. No one wants just another 'also-ran' widget; they want a better widget. So what will make *your* widget stand out ahead of all the others?
Gemmologists, many of whom make their living from the identification and grading of fluorescent and other coloured minerals, do not ever consider testing for UV LW/SW fluorescence as more than a supportive test that may confirm a specimen ID that has been primarily determined by other and much more reliable testing methods. "