Gemstones characteristics

A homogeneous mixture of two or more components, whether solid, liquid, or gaseous, is called a solution. Solutions have variable composition while pure substances do not. That is, the relative amounts of the various components in a solution can vary. Thus, air, salt water, and sixteen carat gold are each solutions. The gemstone, ruby, is also a solution since it consists of the mineral corundum (AI2O3) with some of the aluminum replaced by chromium to give the crystal its characteristic color. Since the amount of chromium present can be varied, ruby is a solution. 

In addition to the minerals, there are also some rock-forming homogeneous materials that have neither the definite chemical composition nor the distinctive crystal structure characteristic of minerals. Such materials cannot, therefore, be considered as minerals and are known as mineraloids. Obsidian, for example, a natural material that has been widely used since prehistoric times for making lithic tools and decorative objects, is a mineraloid. Obsidian has neither a definite chemical composition nor a characteristic crystal structure and is not, therefore, a mineral. Copal and amber are other mineraloids that since antiquity have been treasured as semiprecious gemstones. 

Other optical properties of gemstones, which also determine their beauty and other characteristics that make some of them unique, include the way they disperse light incident on them (see Textbox 22), their refractive index, which is unique to, and characteristic of every type of gemstone and is often used for their identification (see Textbox 22), and their luster, adularescence, asterism, and brilliance. 

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 addition to silica (silicon dioxide SiO ), the crystal form of silicon is found in several semiprecious gemstones, including amethyst, opal, agate, and jasper, as well as quartz of varying colors. A characteristic of quartz is its piezoelectric effect. This effect occurs when the quartz crystal is compressed, producing a weak electrical charge. Just the opposite occurs when electric vibrations are fed to the crystal. These vibrations are then duphcated in the crystal. Quartz crystals are excellent timekeeping devices because of this particular characteristic. 

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. 

Figure 15.1 shows the three ways the atoms of a crystalline solid can be arranged. As a molecule goes from a simple cubic structure to a face-centered cubic structure, the density increases. The less space between the atoms, the more tightly packed the entire molecule, and the harder and less flexible. Unlike amorphous solids, a lattice structure provides for predictable breaks along set lines. This is the reason why diamonds and gemstones can be cut into facets. The round, oval, pear, emerald cut, and diamond-shaped cuts used in jewelry can be cut by dilferent gem cutters all over the world due to their characteristic lattice structures. 

Gemstones are identified by the foiiowing characteristics (known as the four C s). 

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 mounting medium may display its own characteristic spectrum, which may interfere with the spectrum of the stone to be analyzed, especially if the stone is very small. Blu-Tack, for example, exhibits a high Raman fluorescence, which may subdue the Raman spectrum of the gemstone under analysis. 

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