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On September 17th, 1998 the Ithaca Journal ran its first "Ask A Scientist!" article in which Professor Neil Ashcroft , who was then the director of CCMR, answered the question "What is Jupiter made of?" Since then, we have received over 1,000 questions from students and adults from all over the world. Select questions are answered weekly and published in the Ithaca Journal and on our web site. "Ask A Scientist!" reaches more than 21,000 Central New York residents through the Ithaca Journal and countless others around the world throught the "Ask a Scientist!" web site.

Across disciplines and across the state, from Nobel Prize winning scientist David Lee to notable science education advocate Bill Nye, researchers and scientists have been called on to respond to these questions. For more than seven years, kids - and a few adults - have been submitting their queries to find out the answer to life's everyday questions.

Previous Week's Question Published: 6 August, 2008 Next Week's Question
Impurities in diamonds, real and 'fake,' give them fluorescence
Question
When diamonds are put under a black light why do some glow and some don't? Do the real ones glow or is it the fake ones that glow, or are they all real and glow differently because of things like quality clairty, shape, cut, or things like that?

Question
The short answer is that the glowing that you see under black, or ultraviolet light, is called fluorescence. It is the same process that goes on in your fluorescent light bulbs at home. The fluorescence is due to the existence of an impurity in the diamond, that allows the diamond to absorb ultraviolet light which you cannot see, and emit visible light. Such impurities can exist in both natural and artificial diamonds, and do not depend on the shape or cut of the crystal. In the case of artificial diamonds, the method of growth dictates both the type and number of impurities that are present. Generally speaking, no crystal is pure, and artificially grown diamonds will have a distinct impurity signature, which will be fairly uniform while natural ones will be more random. Some impurities will fluoresce under ultraviolet light, others will not. The clarity of the diamond is a measure of the number of impurities, as a pure diamond would be completely clear.

Now I will try to give a more complete answer regarding what is going on. In order to do this, I need a few concepts, which I will introduce now. Light can be thought of as being made up of particles called photons. These photons have different energy for all the different colors of light. Red light has a smaller energy than violet light because red light has a longer wavelength. Ultraviolet light has a shorter wavelength and more energy per photon than any kind of visible light. In general the energy of a photon is proportional to 1/wavelength.

The next concept we need to understand is that atoms can emit light through electronic transitions. This happens when an electron that is not in the lowest possible energy configuration, 'jumps' to the lower energy state. When this happens the atom emits a photon, the color of which corresponds to energy of the jump or transition.

With these two concepts in hand we can return to the diamond. Diamond is a crystal form of carbon. Crystals are simply a regular, predictable arrangement of atoms. This regular arrangement of atoms gives rise to many special properties. In certain crystals, including diamond, the atom's arrangement and electrical interactions give rise to a forbidden range of energies where transitions are not allowed. Pure diamond is clear because the forbidden energy range includes the energies which correspond to photons in the visible range. Because the visible light cannot be absorbed it passes though and the diamond is clear. All visible color in diamonds is therefore due to impurities which create isolated energy levels in this forbidden range via their electrical presence. When the crystal fluoresces, what happens is that an electron in a low energy state absorbs a photon and ends up in a higher energy state. It then loses some of that energy to processes that generate heat. Finally, it gives up its remaining energy to a photon in the visible range, which is the light that you see, when it glows.