Gemstones impurities

Several gemstone species occur in various colors, depending on the presence of impurities or irradiation-induced color centers. Examples are the beryl, comndum, and quart2 families. Quart2 has poor optical properties (RI = 1.55, DISP = 0.013), but becomes of gemological interest when it exhibits attractive colors. Any material can have its color modified by the addition of various impurities synthetic mby, sapphires, and spinel are produced commercially in over 100 colors (2). Synthetic cubic 2irconia has been made in essentially all colors of the spectmm (11), but only the colorless diamond imitation is produced commercially in any quantity. 

The two extremes of ordering in solids are perfect crystals with complete regularity and amorphous solids that have little symmetry. Most solid materials are crystalline but contain defects. Crystalline defects can profoundly alter the properties of a solid material, often in ways that have usefial applications. Doped semiconductors, described in Section 10-, are solids into which impurity defects are introduced deliberately in order to modify electrical conductivity. Gemstones are crystals containing impurities that give them their color. Sapphires and rubies are imperfect crystals of colorless AI2 O3, red. 

Ruby and Sapphire. Ruby and sapphire are "sister stones" both are gemstone forms of the mineral corundum (composed of aluminum oxide). Pure corundum is colorless, but a variety of trace elements cause corundum to exhibit different colors. Ruby is red corundum, while sapphire is corundum in all colors except red. The red in rubies is caused by trace amounts of chromium the more intense the red color of a ruby, the more chromium it contains. The blue in sapphires is caused by titanium and/or iron impurities (Garland 2002 Hughes 1997). 

In the case of boron impurities a complementary situation occurs. Boron has only three outer bonding electrons instead of the four found on carbon. Each boron impurity atom occupies a carbon position, forming Be, which results in the creation of a set of new acceptor energy levels just 0.64 x 10 19 J (0.4 eV) above the valence band. The transition of an electron from the valence band to this acceptor level has an absorption peak in the infrared, but the high-energy tail of the absorption band spills into the red at 700 nm. The boron-doped diamonds therefore absorb some red light and leave the gemstone with an overall blue color. 

Pigments, minerals, gemstones, glasses, and many related materials are colored by impurity defects that absorb some of the incident white light, leaving a depleted spec-hum to color the solid. Colors in these materials are thus characterized by the absorption spectrum of the solid. Common inorganic colorants are the transition-metal and lanthanide metal ions. The colors ate characteristic of the ions themselves and are due... 

In a perfect crystal, all atoms would be on their correct lattice positions in the structure. This situation can only exist at the absolute zero of temperature, 0 K. Above 0 K, defects occur in the structure. These defects may be extended defects such as dislocations. The strength of a material depends very much on the presence (or absence) of extended defects, such as dislocations and grain boundaries, but the discussion of this type of phenomenon lies very much in the realm of materials science and will not be discussed in this book. Defects can also occur at isolated atomic positions these are known as point defects, and can be due to the presence of a foreign atom at a particular site or to a vacancy where normally one would expect an atom. Point defects can have significant effects on the chemical and physical properties of the solid. The beautiful colours of many gemstones are due to impurity atoms in the crystal structure. Ionic solids are able to conduct electricity by a mechanism which is due to the movement of fo/ 5 through vacant ion sites within the lattice. (This is in contrast to the electronic conductivity that we explored in the previous chapter, which depends on the movement of electrons.)... 

Several gemstone species occur in various colors, depending on the presence of impurities or irradiation-induced color centers. Any material can have its color modified by the addition of various impurities synthelic ruby, sapphires, and spinel are produced commercially in over 100 colors. 

Aluminum occurs in many common minerals and clays, as well as in gemstones. Sapphire and ruby are both impure forms of AI2O3 and receive their color from the presence of small amounts of other elements (Cr in ruby Fe and Ti in sapphire). Most aluminum is currently obtained from bauxite, A1203 H20, which occurs in large deposits in Australia, the United States,... 

But gemstone color is due to very small impurities in the mineral. For example, the mineral known as corundum is colorless when pure. But a very small amount of chromium produces a bright red color. The corundum is now a ruby. A touch of iron or titanium produces shades of yellow, green, purple, pink, or blue that turn it into a sapphire. 

Two gemstones are made primarily of beryl. They are emeralds and aquamarines. In emeralds, traces of chromium produce a brilliant green color. In aquamarines, iron is the impurity. It gives the beryl a beautiful blue color. 

Crystals have fascinating properties. A clear, colorless quartz crystal is pure silicon dioxide (Si02). But a crystal that is colorless in its pure form may exist as a variety of colored gemstones when tiny amounts of transition element compounds, usually oxides, are present. Amethyst (purple), citrine (yellow-brown), and rose quartz (pink) are quartz crystals with transition element impurities scattered throughout. Blue sapphires are composed of aluminum oxide (AI2O3) with the impurities iron(II) oxide (FeO) and titanium(IV) oxide (Ti02). If trace amounts of chromium(III) oxide (Cr203) are... 

The name and symbol come from the Persian word zargun, meaning gold colored. Various forms of zirconium have been known in the form of gemstones since ancient times, but it was Klaproth who noted an unidentified substance in a jargon stone. The elemental metal (in impure form) was isolated in 1824 by Berzelius. Zirconium has been found in stars, meteorites, and lunar rocks. It exists as a compound widely in nature but is usually found in quantities too small to be of commercial interest. Zirconium is used in specialized laboratory equipment, explosives, and lamp filaments and in lotions to treat poison ivy. In the form of zirconium oxide, it is used to make gems and for specialty glass. 

Because of its rarity, beryllium has only a few practical uses. It occurs mainly as beryl, Be3Al2SigOjg, a gemstone which, with appropriate impurities, may be aquamarine (blue) or emerald (green). Because it is transparent to X-rays, windows for X-ray tubes are constructed of beryllium. Beryllium compounds are quite toxic. 

In 1985 benitoite became the California state gemstone. Found only in a tiny mine near Coalinga, California, it is a silicate of harium and titanium with trace impurities that cause a range of hues from colorless to blue to pink. Its general formula is BaTi(Si03)3. If a 15-carat stone were pure BaTi(Si03)3, how many moles of silicon would it contain ... 

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