Transition metals gemstones

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... 

Another example of the coloration phenomenon is demonstrated by transition metal doping of crystals exhibited by gemstones such as ruby. Table 2.6 lists some common gemstones, and the respective host crystal and dopants that give rise to their characteristic colors. Whereas crystals of pure corundum (a-alumina) are colorless. 

Why do the transition metals impart color to gemstones such as emeralds and rubies ... 

The beautiful pure color of gems, so valued by cultures everywhere, arises from trace transition metal ion impurities in minerals that would otherwise be colorless. For example, the stunning red of a ruby, the most valuable of all gemstones, is caused by... 

Transitions between orbitals localised on atoms e.g. d-d transitions of transition metal salts, f—f transitions of lanthanide ions. Such metal-centred (MC) transitions are ubiquitous in transition metal and lanthanide complexes. They are relatively weak because they are symmetry (Laporte) forbidden. Although they may not be the important transitions for any particular application of transition metal photochemistry, they will almost always be present. These are the transitions that give many transition metal salts their characteristic colour and are foimd in some gemstones and minerals. For example, the red colour in ruby is due to the d-d transitions in chromium (III) present at certain sites in an aluminium oxide (corundum) crystal. 

The colors of many gemstones are due to transition-metal-ion impurities in the mineral. Ruby has Cr in alumina, AI2O3, and emerald has in beryl, BejAlzlSiOjle. 

The color of a gemstone is determined by the presence of small amounts of one or more transition metals. For example, aluminum oxide, AI2O3, often occurs naturally as corundum—a clear, colorless mineral. However, if as few as 1 to 2% of the aluminum ions, Al " ", are replaced by chromium ions, Cr " ", the corundum takes on a reddish color and is known as the gemstone ruby. 

Table 3A on the next page lists transition metals that are responsible for the colors of various gemstones. The table provides only a general overview, however, as most naturally occurring gemstones occur in a range of hues, depending on the exact composition of the stone. 

Verneuil s method, although somewhat modified, is still widely used today for the manufacture of colored gemstones. When magnesium oxide is substituted for aluminum oxide, a colorless spinel-like product is formed. The addition of various transition metals then adds a tint to the spinel that results in the formation of synthetic emerald, aquamarine, tourmaline, or other gemstones. Synthetic gems look very much like their natural counterparts. 

The colors of several other gemstones are also caused by the splitting of the d orbitals in transition metal ions imbedded within host crystals. For example, the red in garnet, which has Mg3Al2(Si04)3 as a host crystal, and the yellow-green of peridot, which has Mg2Si04 as a host crystal are both caused by electron transitions between d orbitals in Fe. Similarly, the blue in turquoise, which has [Alg(P04)4(0H)g 4 H20] as a host crystal, is caused by transitions between the d orbitals in Cu. ... 

In this chapter, we examine the properties of the transition metals and their ions more closely. We also examine the properties of coordination compounds in some detail. We first discussed this common type of transition metal compound in Chapter 16 (see Section 16.8). In a coordination compound, bonds to a central metal ion split the d orbitals much as they are split in the aystals of gemstones. The theory that explains these splittings and the corresponding colors is crystal field theory, which we also explore in this chapter. 

Corundum, when pure, is also known as the gemstone white sapphire. Certain other gemstones consist of corundum with small amounts of transition metal ions as impurities Cr in ruby and Fe and Ti in blue sapphire, for example. Artificial gemstones are made by fusing corundum with carefully controlled amounts of other oxides. 

Transition metal ions are responsible for the brilliant colors in many gemstones. For a discussion of the ions responsible for the colors of rubies and emeralds, and the mechanisms that allow these gemstones to maintain their colors indefinitely, go to the Focus On feature for Chapter 24, Colors in Gemstones,... 

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