In their classic book Gemology (Wiley, 1979), the mineralogists Cornelius Hurlbut of Harvard and George Switzer of the Smithsonian Institution claim, "Since there is no rigid set of criteria that separate gems of great value from those of less value, the term semiprecious should be abandoned and all gems referred to as precious." However, since many people still divide gems between the two types, we offer here definitions of precious and semiprecious stones from The Glossary of Geology, 3rd Edition, published by the American Geological Institute:
"A gemstone that, owing to its beauty, rarity, durability, and hardness, has the highest commercial value and traditionally has enjoyed the highest esteem since antiquity; specifically, diamond, ruby, sapphire, and emerald (and sometimes pearl, opal, topaz, and chrysoberyl)."
"Any gemstones other than a precious stone, or any gemstone of lower commercial value than a precious stone; specifically, a mineral that is less than 8 on the Mohs scale of hardness. A gemstone may also be regarded as semiprecious because of its comparative abundance, inferior brilliance, or unfamiliarity to the public, or owing to the whims of fashion. This arbitrary classification is misleading, as it does not recognize, for example, that a ruby of poor quality may be far less costly than a fine specimen of jadeite."
Gemstones gain their color from the way they affect light as it passes through them. Like a prism, a clear diamond splits incoming light into its constituent wavelengths, creating a pleasing rainbow of colors. Many other gems get their distinctive colors by absorbing one or more wavelengths, because of their chemical compositions. Rubies and sapphires, for example, are both varieties of the mineral corundum, identical in almost every respect. The difference lies in the trace amounts of other elements present. A dash of chromium makes a vivid red ruby; a bit of iron and titanium results in a deep blue sapphire.
Gemstones have specific chemical compositions that serve to identify them. Diamond consists of carbon, for instance, while emerald is a beryllium aluminum silicate and peridot is a magnesium iron silicate. Some gemstones, such as jade, are aggregates of one or more minerals.
Figure 1. Crystals of the cubic system: a cube (e.g., pyrite) at left, and an octahedron (e.g., diamond) at right. Note the three crystallographic axes, which meet in the center, are of equal length.
All crystalline minerals consist of atoms packed in geometric arrays; the arrays, called crystal structures, are divided into seven crystal systems based on the symmetry of their atomic geometries. Mineralogists distinguish among the different crystal systems by drawing imaginary lines called crystallographic axes between the points, joints, or planes of the crystals. These lines intersect in characteristic ways at a point within the crystal called the origin. Crystals in the cubic system, for example, have three crystallographic axes, all of equal length and all at right angles to one another. (Note that cube-shaped crystals are not the only kind in the cubic system; other shapes, including octahedrons and icositetrahedrons, also qualify, because they have crystallographic axes that meet the definition.)
Figure 2. Crystals of the tetragonal system: a bipyramid at left and a prism and two bipyramids (e.g., zircon) at right. Note the vertical crystallographic axis differs in length from the two horizontal axes.
A second type of crystal system, known as tetragonal, also has three crystallographic axes at right angles to one another. However, while two of the axes are of equal length, the third is either shorter or longer. The bipyramid, for example, is an elongated octahedron.
The five other systems are hexagonal, trigonal, orthorhombic, monoclinic, and triclinic. Along with composition, crystal structure determines a mineral's properties. It also influences what happens to light when it enters a mineral and where planes of weakness may lie, which helps gemcutters.
Hardness, which mineralogists can use to help identify minerals, is a measure of a gemstone's resistance to abrasion. In 1822, the Austrian mineralogist Friedrich Mohs developed the Mohs scale, a standard scale using 10 familiar minerals against which all minerals are measured. A mineral (or, on the scale below, an everyday object) will scratch all minerals of a lower number on the scale. The intervals between minerals on the scale are not uniform, however. For instance, the difference in hardness between diamond and sapphire is much greater than that between sapphire and topaz. The minerals, from least to most hard and interspersed with several everyday objects for comparison, are as follows:
Origin of Name
The origins of the names of many gemstones have come down to us from antiquity and in many cases are somewhat obscure, but in those cases we offer the best guesses of historians.
Specific gravity, a property that mineralogists use to identify minerals, is the number of times heavier a gemstone of any volume is than an equal volume of water. That is, it is the ratio of the density of the gemstone to the density of water.