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HIGH-SCHOOL CHEMISTRY
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The Cause of Color in Precious Stones FRANK B. WADE Shortridge High School, Indianapolis, Indiana
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N USING modern physics to account for the beautiful colors of some of our precious stones we are dealing with only a minor part of a much larger subject, namely, the emission and absorption of electromagnetic radiations by matter. We shall be interested only in the spectral lines which the human eye can recognize, and shall confine ourselves to those gems which have what is called allochromutu character rather than ideochromatic properties. Stones of the transparent type, such as rubies, sapphires, emeralds, amethysts, etc., come under this cl&iification. It is now -rather well established that the ootical &ects of matter reside mainlv in the outer two enerw -levels of the atom,' corresponding to the valence electron levels of the chemist. Expressed in electronvolts, the amount of energy required to affect the electrons of these levels is relatively small. Therefore, in constructing a theory of the cause of color in gems we must seek facts about the valence electrons of those atoms concerned with the appearance of color. For a long time it has been increasingly apparent: (1) that the reported impurities of color in otherwise colorless materials, such as white sapphire, crystal quartz, colorless beryl, and others, were elements in the so-called "transition" series in our periodic table (the elements of atomic numbers 22 to 29, i. e., titanium, vanadium, chromium, manganese, iron, cobalt, nickel, and copper); (2) that these are the elements which give us most of our colored ions (the ionic condition may be necessary) ; and (3) that these elements are among those most used to make colored glass. Here we have some facts which seem to connect color in transparent media with the elements of the transition series. Again, i t is notably true that changes in the valence of these elements, as ferrous to ferric, chromous to chromic, cobaltous to cobaltic, etc., are usually accompanied by a change of color in the ions concerned. Supporting facts come from gem lore. Aluminum oxide (sapphire) or magnesium aluminate (spinel) are the basic materials in the manufacture of all synthetic 1 Dn BROOLIE."Atomes. Radioactivith. Transmutations." Flammarion. ~diteur.Paris, France, 1937. De Bro& insists that chemical properties, and phenomena in o f vkihle lieht and cohesion concern onlv the elec. .. the .... realm . .trons in the most exterior levels. where in the case of aliatoms the labor of extraction dws not su6ass more than a relatively small number of electron-volts.
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gems. Both of these are colorless unless traces of the transition elements are supplied-chromium for ruby, iron and titanium for blue sapphire, nickel for a yellow stone, cobalt in spinel for blue, vanadium in either sapphire or spinel for a changeable stone which is green in bright daylight but reddish in artificial light of long wave length, and which is sold as synthetic Alexandrite. (The true Alexandrite is a chrysoberyl of similar color tendencies.) Moreover, during the manufacturing process, it seems necessary to maintain reducing conditions in the furnace to obtain the desired color of synthetic blue sapphire. Again we have a valence factor involved, as well as the presence of two transltion elements, iron and titanium. Further evidence of a low order of energy such as is found among the valence energy levels of atoms is the fact that a ruby becomes green when it is gently heated and stays green while hot, but returns to its red color when i t is cooled. The slight addition of energy required to raise the temperature a few hundred degrees changes the color of the stone. More evidence may be had from a report of Dr. George Clark of the University of Illinois, in the Illinois Science Teacher of October and December, 1940, that an x-ray examination of zinc oxide a t 600°C. reveals breaks in the space lattice--evidence of the loss of electrons. At this temperature the zinc oxide also turns yellow and becomes a conductor of electricity. On receiving a moderate amount of heat energy, zinc becomes capable of resonance effects as far as visible light is concerned. The fact that the natives of Thailand can alter the color of brown zircons simply by the manner of heating is another point to make in connection with the importance of valence electron levels of gems. When the brown stones are heated in a reducing atmosphere, while buried in charcoal, they turn blue, and when they are heated while exposed to the open air, they turn white. Now that we have sufficient evidence of a fundamental connection between the electrons in the valence levels of the elements of the transition series and the colors of allochromatic gems, colored glasses, and solutions which contain colored ions, let us attempt an explanation. The late Sir William Bragg, in "The Universe of Light" (p. 124) says, "The molecular absorption of light is of the nature of a resonance."
We know that the color of indigo can be changed from blue to colorless by a very slight change in its molecule, for example, the addition of two atoms of hydrogen. The blue color can be restored by a mild oxidation which removes the two hydrogen atoms. Thus, we may thimk of the absorption of visible light which occurs in gems as a resonance eiTect among the valence electrons of the atoms of the coloring impurities, and conclude that the energies, or frequencies, of the outer electron level of the eight elements which have been mentioned as causing the colors, are in tune with the energies, or frequencies, of some part of visible light. Part of the light is transformed-perhaps into heat-and part succeeds in penetrating into the atomic maze, the resulting color depending on the element involved. In the case of colorless minerals, none of the light energy is removed by vibrating "sympathetically" within the mineral. All the energy in the visible spectrum comes out equally well, and white light is not absorbed by the material. If a stone contains more than a small amount of a coloring agent, i t appears black. The coloring agent must be in a condition corresponding to dilute solution in order to pick up the specific enerRy and transmit the rest of the light. our theory, then, is that color in gems is due to
selective absorption of parts of the visible frequencies of light by a resonance effect among the valence electron levels of particular atoms. These atoms, most often found in the transition series, are present only in small amount, i. e., in low concentration in the space lattices of the crystals. Probably effects similar to what we call color occur when electromagnetic energy of frequenaes unrecognizable to the human eye passes through layers of other elements, thus being sympathetically absorbed or passed on, according to resonance effects. There are many optical effects involved in rationalizing the many interesting behaviors of gems. Dichroism, the differential absorption in different directions in some crystalline materials, is accompanied by plane polarization of light. Fluorescence, with its accompanying change of frequency, usually but not always toward lower frequency, must be accounted for; also phosphorescence, a slow dying-out of the disturbed condition after the source of provoking energy has been cut off. Refraction and rdection still must be accounted for on a modern basis. It is the author's hope that this attempt to rationalize color effects in gems may lead some better-prepared scientists to apply their physics and mathematics successfully to thisinteresting problem.
