All That Glisters: Behind the Sparkle of Gems
Several months ago, my sister and I were at a jewelry store in the mall. The immense variety of jewels in a small store in the mall was staggering. Diamonds, rubies, and emeralds, among many other minerals, sparkled in settings designed to show them at their best. While she was thinking about how pretty the pieces were, I was thinking chemistry. Different stones have different chemical makeups. Some are differentiated by minute impurities, while others have nearly nothing in common, leading to differently-shaped crystals and different shapes for cutting. The atoms that make up the stones are arranged in different ways that give them their particular qualities. Take a look and be impressed.
The amethyst derived its name from the Greek combination of a (not) and methustos (drunk) because, in ancient times, it was thought to protect against drunkenness. It’s a good thing it looks beautiful, because I doubt it fulfilled that purpose. Amethyst is a variety of quartz, so it is made from tetrahedra (four-member pyramids) of SiO4. Each oxygen is shared between two tetrahedra, so the overall formula is silicon dioxide, SiO2. The lovely purple color comes from substituting Fe3+ (Iron missing three electrons) in certain places for silicon. There is also a bit of red in an amethyst due to the presence of manganese. This two-color property is called dichroism. When amethyst is heated, it changes color to yellowish brown but loses the dichroism. The purple color can fade if the stone is overexposed to light.
Topaz has the formula Al2SiO4(F,OH)2 and is therefore classified as a silicate. It has nice, rectangular crystals. The pure stone is clear but tinted by impurities. The traditional topaz is orangeish brown. Sorry, poor souls born in November; I think you got saddled with an ugly stone.
There are, however, other color assortments too. Topaz can come in blue and a variety called Imperial topaz, which is yellow, pink, or yellow-orange. Some stones turn pink upon heating due to the presence of chromium (Cr3+). Other colors are produced by hole color centers, which are often affected by the varying amounts of F- (fluoride) and OH- (hydroxide). A hole color center happens when another element with a lesser charge is substituted for the silicon. This places more stress on the oxygen, so one of its electrons is more easily stripped off by radiation. The remaining electron has access to different energy levels, and its actions produce the color. Because of these color centers, topaz can be colored by irradiation.
One of my favorite gems is the sapphire because of its beautiful deep blue color. It has a simple formula: Al2O3, aluminum oxide, also called corundum. It’s a hard stone, second only to diamond’s perfect 10, as ranked on the Mohs scale of hardness. Sapphires come in every color except red. Non-blue sapphires always have their color preceding the word sapphire, like green sapphire. However, this nomenclature is fairly recent; old mentions of sapphires refer always to the blue gem. The blue color is produced by trace substitutions of titanium and iron, with amounts as low as 0.01 percent. This gives the stone a deep blue color because of the transfer of an electron from the iron to the titanium. This changes the electron’s energy and absorbs energy equivalent to yellow light. When subtracted from white light, the blue color results. Pale green, yellow, and brown sapphires are colored by iron, and with very small amounts of chromium, pink sapphires are made. Beside jewelry, sapphires, usually synthetic, are used in sodium vapor lamps or circuits to conduct away heat.
Rubies are nearly the same as sapphires because they, too, are colored corundum. Rubies have use outside of gemstones. Theodore Maiman used ruby to make the first laser, and ruby lasers are still in use today. Because of their hardness, rubies are used in the workings of modern clocks. The red in ruby comes from substitution of chromium (Cr3+) for aluminum in the crystal lattice. There is some overlap with sapphire in this case. This coloration occurs with as low a substitution as 1 part in 5,000; the substitution occurs in the same manner as illustrated above. The crystals are hexagonal prisms and often under 10 carats due to the presence of Cr3+. Stones can have a star in them due to the presence of TiO2 (titanium oxide) crystals lying in the same direction as crystal growth. This is displayed when the stone is cut as a cabochon, with a round top and flat bottom.
Emeralds are a special form of beryl, Be3Al2Si6O18 and grow in hexagonal crystals. The famous green color is due to the presence of trace Cr3+. Emeralds aren’t very sturdy; they often have inclusions of other minerals, internal abnormalities, or surface fissures that weaken the structure. This happens so often that emeralds are graded by eye rather than under magnification, as diamonds are. The weakness also led to the development of the emerald cut, which minimized loss of material and maximized appearance by working with the crystal structure. Knowing what we do about the structure of the emerald, we shouldn’t be surprised to know that emeralds are twenty times rarer than diamonds. However, only the finer cuts have a similar market value as diamonds.
Alexandrite is the gemstone variety of the mineral chrysoberyl,BeAl2O4 . The color in the stone is due to Cr3+ substituting for aluminum in the crystal. Its claim to fame is its ability to change color depending on light source. In sunlight, it is bright green. Under a tungsten lamp, it turns a deep red. The change occurs due to how Cr3+ absorbs and emits light. The absorption of Cr3+ in rubies occurs at 550 nanometers (nm), while in emeralds, the band is at 600 nm. The band in alexandrite is located at nearly the middle of the range, 580 nm. Sunlight contains higher proportions of blue and green light, while a tungsten bulb has a greater proportion of red light. Because the stone’s Cr3+ absorption is in the middle, the stone’s color depends on how much of what color of light is emitted. Daylight contains more green light, so the stone reflects green when in the sun.
Many minerals are used in jewelry. I covered only a few of the more popular ones, but mineral identification guides list dozens more. As I researched them, I discovered that despite striking differences in appearance, they are similar in other ways. For example, Cr3+ is a coloring agent in many of the above stones. Aluminum oxide provides a large part of the framework of many of these. Topaz and emerald are both classified as silicates, but topaz is less likely to be scratched or shattered. The next time you see precious stones, think about all the molecular madness that happens to make the stones’ macroscopic beauty.
Bonewitz, Ronald. Rock and Gem. New York: DK, 2005. Print.
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