Spotlight on the gem world: Glowing diamonds.The Hope diamond shown here is one of the most famous d

Spotlight on the gem world: Glowing diamonds.The Hope diamond shown here is one of the most famous diamonds on Earth, and certainly the most visited. It has had a fascinating journey through history, from its purchase at the Golconda mines of India by Jean Baptiste Tavernier, through its time as the great blue diamond of the French crown followed by its theft and recutting from 112 to 67b carats (0.2 grams) weight during the revolution and its final journey to the Smithsonian. It also provides a spectacular illustration of two of diamond’s intriguing properties: fluorescence and phosphorescence.Diamonds are coloured by impurities, and most have a slight tinge, usually yellow or brown. These impurities have loose electrons that absorb some of the wavelengths of light, leaving the residual light we perceive with the eye. In blue diamonds like the Hope, boron removes many yellow wavelengths, while in the more common yellowish diamonds, nitrogen removes blue. These wavelengths of light also have different energies, which is why they are split out of white light by a prism. All you need to remember for this introduction is that violet has the highest energy in the rainbow, and red the lowest.Fluorescence occurs because some of the electrons get excited by absorbing light, especially at higher energy ultraviolet (UV) wavelengths. These electrons jump up to a higher energy state when excited. They fall back to their initial equilibrium energy state when the excitation, such as a UV light, is switched off. While doing so they give off their energy at a wavelength specific to the impurity’s electron energy jump. This is what we see as fluorescence, and the emitted wavelength is always longer wavelength and lower energy than the exciting one, so invisible (to us) UV light will make a stone fluoresce in a lower energy part of the rainbow. Please be careful playing with UV lights if you try this at home, as the higher energy ones can damage the retina.In the Hope’s case, absorbed UV energy is emitted as red light. In the more common yellow series diamonds the fluorescence is usually blue. There are many other possible colours of glow, green and yellow also being fairly common. Fluorescence is used to make laser light, by exciting a doped crystal or gas, along with the well known strip lights we see every day.While some see fluorescence at detracting from a stone’s beauty and lowering its value, it can have advantages beyond exhibiting this special and beautiful phenomenon. A slightly yellow diamond with a medium-strong blue glow will look whiter in daylight (which contains UV from the sun) than they are, so a stone graded as H-I can look like a F-G. The difference will not be as visible in electric light however, as it contains little UV. On the other hand, such a stone seen in a daylight equivalent lamp in a jewelers can look yellower when you see it at home. A sign of strong blue fluorescence is a slightly hazy or milky looking stone (though microscopic inclusions can also cause this appearance). Fluorescence can sometimes be a pointer to possible treated or synthetic diamonds, which we’ll cover in a future post.Jewellery designers have started incorporating fluorescence in their pieces, choosing stones as for their glow as well as its beauty. When worn in a nightclub with UV lights, this hidden aspect of the jewel becomes visible.I reference links below to three images from the Aurora Pyramid of Hope, part of a private collection of coloured diamonds. These give an overview of the possible shades of diamonds and their varied and individual behavior under UV light. The original is on display at the London Natural History Museum.Phosphorescence occurs when the glow continues after the source of excitation has been switched off. The Hope carries on glowing visibly for 5 minutes after the UV light goes off, and can still be measurable half an hour later. This happens because the electrons accumulate extra energy like a battery and have to give it off until they return to their equilibrium energy state.LozImage credit: Smithsonian Institution.The Hope diamond:http://mineralsciences.si.edu/hope.htmhttp://www.si.edu/Encyclopedia_SI/nmnh/hope.htmhttp://www.sciencedaily.com/releases/2008/01/080104145404.htmFluorescence in diamonds:http://www.truthaboutdiamonds.com/truth-about/diamond-fluorescence/http://lgdl.gia.edu/pdfs/W97_fluoresce.pdfThe Aurora collection images:http://commons.wikimedia.org/wiki/File:Aurora_Diamond_Collection.jpgNear bottom of this page: http://twomonthsineurope.wordpress.com/2012/07/03/natural-history-museum-london/http://en.wikipedia.org/wiki/Aurora_Pyramid_of_Hope -- source link

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