Optical Properties of Gemstones

Optical Properties of Gemstones: Color is the most obvious visual feature of a gem, but in fact, it is just one of many optical properties. All of these are dependent on light. The individual crystalline structure of a gemstone interacts with light in a unique way. That determines the optical properties of each gem species. Effects produced by light passing through a gem are different for every crystal.

What Makes Color?

The color of a gem depends largely on the way it absorbs light. White light is made up of colors of the rainbow (spectral colors), and when it strikes a gem, some spectral colors are ‘preferentially absorbed.’ Those that are absorbed, pass through, or are reflected back, giving the gem its color. Each gem has a unique color ‘fingerprint’ (known as its absorption spectrum). However, this is only visible when viewed with a spectroscope. To the naked eye, many gems look the same color.

Splitting Light Through A Prism

Splitting white light into its spectral colors is called dispersion and gives gems their internal fire.

1. Allochromatic Gems

Allochromatic, meaning other-colored, gems are colored by trace elements or other impurities. These are not an essential part of their chemical composition. Corundum, for example, is colorless when pure. However, impurities in it (usually a metal oxide) create the red stones we know as rubies, green, yellow, and blue sapphires, and orange-colored padparadscha. Allochromatic gems are often susceptible to color enhancement or change.

2. Idiochromatic Gems

The color of idiochromatic, meaning self-colored, gems comes from elements that are an essential part of their chemical composition. Hence, idiochromatic gems generally have only one color or show only a narrow range of colors. Peridot, for example, is always green, because the color is derived from one of its essential constituents, iron.

3. Parti-Colored Gems

A crystal that consists of different-colored parts is called particolored. It may be made up of two colors (bicolored), three (tricolored), or more. The color may be distributed unevenly within the crystal, or in zones associated with growth. The many different varieties of tourmaline probably show the best examples of particolored gemstones. These may exhibit as many as 16 different colors or shades within a single crystal.

Bi-colored crystals can make attractive gemstones; junctions of color zones may be distinct or gradual.

4. Pleochroic Gems

Gems that appear one color from a direction, but exhibit one or more other shades or colors when viewed from different directions, are known as pleochroic. Amorphous or cubic stones show one color only; tetragonal, hexagonal, or trigonal stones show two colors (dichroic); orthorhombic, monoclinic, or triclinic stones may show three colors (trichroic).

Iolite is an example of pleochroic gems. It is strongly pleochroic, i.e., colorless from one direction and blue when rotated 90 degrees.

Refractive Index (RI)

When a ray of light meets the surface of a polished gemstone, some light is reflected, but most passes in. Because the gem has a different optical density from the air, the light slows down and is bent from its original path (refracted). The amount of refraction within a gem is called its refractive index (RI). With the DR, it can be used to help identify the stone.

Calcite is highly birefringent and produces double images. Similarly, zircon’s back facets look doubled, due to strong double refraction (DR).

Birefringence (DR)

When viewed through a refractometer, cubic minerals like spinel are singly refractive. It shows a single shadow edge. Double-refractive minerals like tourmaline split light rays in two, producing two shadow edges. The difference between the two gives the ‘birefringence’ (DR).

Luster

The overall appearance of a gemstone, its ‘luster, is determined by the way light is reflected from its surface. This is related to the degree of polish, which is generally greater for the harder stones. Gemologists use a variety of terms to describe luster and its degree of intensity. ‘Splendent’ means that the stone reflects light like a mirror. However, if little light is reflected, the luster may be described as ‘earthy’ or ‘dull’.

Stones with a luster comparable to diamond are described as ‘adamantine’ and are the most desirable. In fact, most transparent, faceted gems have a glass-like, ‘vitreous’ luster. On the other hand, precious metals have a ‘metallic’ luster. Organic gems, however, show a range, from ‘resinous’ to ‘pearly’ and ‘waxy.’

Hematite crystals, like pyrite and precious metals, display metallic luster.

A waxy luster is commonly associated with turquoise.

The greasy luster of a polished imperial jadeite is comparatively rare.

Amber has a resinous luster and it may occur in a range of lusters, depending on the nature of the gem.

Satin spar gypsum often displays a silky luster.

The glass-like luster of a ruby is the most common for cut stones.

Some gemstone species vary in luster. Garnets, for example, range from the resinous hessonite to the adamantine luster of demantoid. Rough lazulite and howlite have a dull, earthy luster, which is vitreous after polishing.

Interference

Interference is an optical property caused by the reflection of light off structure within a gemstone. This internal reflection gives a play of color. In some stones, it will produce the full range of spectral colors. In others, just one color may predominate the gemstone.

Iridescence

Interference in opal is because of the structure of the stone – spheres arranged in regular three-dimensional patterns.

This produces the rainbow effect called iridescence, also shown by a number of other gems such as hematite, labradorite, and iris quartz. In moonstone feldspar, interference at the junctions of its internal layers (thin, alternating layers of different types of feldspar) produces a shimmering effect just below the surface of the stone. It is known as adularescence, opalescence, or a schiller (sheen).

Cat’s-Eyes and Stars

When a gemstone is cut en cabochon (with a domed and polished surface), light reflecting from the stone’s internal features, such as cavities, or fibrous needle-like inclusions, may create a cat’s-eye effect (chatoyancy) or star stones (asterism). One set of parallel fibers gives rise to the cat’s-eye effect. Similarly, two sets of fibers produce a four-rayed star, three sets of fibers a six-rayed star, and so on.

A blue sapphire may display a star effect whereas some chrysoberyl stones show a cat’s-eye effect.