Visual properties
How a gem interacts with light is the
very essence of its nature. Light is the source of a gemstone’s beauty, color,
and sparkle it is also a useful tool for the identification of gems since each
stone has its own particular set of optical properties. For example, there are
a dozen or more red gemstones, and many of the red hues within each type of
stone will have many different shades. All these properties are a way of
identifying gemstones, although no single one is diagnostic in itself. Some
categories such as luster are subjective observations; others, such as a
mineral’s refractive index, are objective. A gemologist identifying a stone
will use a number of different methods and instruments to narrow the
possibilities. Examination of one or all of the optical properties of the stone
will show how it transmits, bends, and reflects light—just one of these may
suffice for identification; in other instances, a complex combination of
physical and optical properties may be needed.
Color
One of the most desirable qualities in a gemstone is beauty, and an important part of this is the stone’s color. In gems, color is caused when light is absorbed within the crystal, or refracted—changing direction as it passes through the gem. White light is composed of many colors; when one or more of those colors is absorbed, the remaining light emerging from the gem is colored. This can be brought about by the presence of trace elements that cause certain wavelengths to be absorbed or can be a part of the gem’s chemical structure.
- Idiochromatic gems
Idiochromatic gems are those sometimes described as “self-colored,” as their color is inherent in their chemical makeup. Rhodochrosite is a manganese carbonate with a natural pink to red color due to its manganese content, and peridot is an iron magnesium silicate, which is green as a result of its iron content.
- Allochromatic gems
Allochromatic gems are those colored by trace elements in their structure. Amethyst and ruby are examples of these: amethyst is colorless quartz made purple by traces of iron, while ruby is corundum colored by traces of chromium.
- Particoloring Gems
Particoloring Gems with different colors within the same stone are called particolored. Gems with two colors are called bicolored; those with three colors, are tricolored. Rarely, a dozen or more colors can occur. Divisions between the colors can be abrupt or gradual. Part-coloring is often caused by changes in the chemical medium in which the crystal has grown.
- Pleochroic gems
Pleochroic gems As white light passes through a gemstone, colors are absorbed differently in different directions: as a consequence, a stone can be a different color when viewed from different angles. This effect is called pleochroism, and it can be an important aid in the identification of cut stones.
Luster
A gem’s luster is the general
appearance of its surface in reflected
light. There are two basic types of luster: metallic and nonmetallic. Precious
metals have metallic lusters, and gemstones are nonmetallic, with the exception of
a few like hematite and pyrite. Lusters that relate to gems include vitreous,
waxy, pearly, silky, resinous, greasy, earthy, metallic, and adamantine.
- Adamantine luster
Gems that demonstrate extraordinary brilliance and shine have an adamantine luster. It is a relatively uncommon luster, possessed by diamonds, some zircons, and very few other gems.
Refractive index
When light passes through a
transparent gem, it changes speed and direction. This is called refraction. The
change in the speed of light as it passes from the air into a gem is called the
refractive index (RI). The change in direction, or bending, of the light, can be
used to calculate the gem’s RI. Diamond’s high RI results in flashes of light
seen when the gem is moved— its “fire.” The greater the dispersion of the white
light, the greater the fire.
- Double refraction
Gemstone minerals in the cubic system bend light equally in all directions. Other types of crystal systems bend light in two directions, with the crystal structure causing light to bend at two different angles. This is called double refraction.
Spectroscopy
The study of the emission of light according to its wavelength is called spectroscopy. Devices known as spectroscopes are used to measure light waves as they pass through gemstones. The spectroscope has a small slit for light to pass through. When a gem is placed between a light source and the slit, a light spectrum is produced. Dark bands appear where certain wavelengths are absorbed by the stone. These bands are characteristic of various elements, enabling the identification of the gem’s chemical makeup. The three spectra on the right reveal much about the gems’ composition.
- Spectra
Ruby’s simple composition shows only a few dark bands or lines. Almandine, meanwhile, shows numerous lines, due to its complex composition, while glass is made of only two elements, so displays only two absorption areas.
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