Gemstones

I

Introduction

Gemstones, minerals that are treasured for their beauty and durability. A large number of minerals have been used as gems. Their value generally depends on four features: the beauty of the stone itself; its rarity; its hardness and toughness; and the skill with which it has been cut and polished. Stones such as diamonds, rubies, and emeralds represent one of the greatest concentrations of money value. During times of war or economic disturbance many people convert their wealth into precious stones, which are transportable and more easily sold.

II

Optical Properties

The beauty of gems depends to a large extent on their optical properties. The most important of these are the degree of refraction and colour. Other properties include fire, the display of prismatic colours; dichroism, the ability of some gemstones to present two different colours when viewed in different directions; and transparency. Diamond is highly prized because of its fire and brilliance, ruby and emerald because of the intensity and beauty of their colours, and star sapphire and star ruby because of the star effect, known as asterism, as well as for their colour.

In certain gemstones, notably opals, brilliant areas of colour can be seen within the stone; these areas change in hue and size as the stone is moved. This phenomenon, known as play of colour, differs from fire and is caused by interference and reflection of light by tiny irregularities and cracks inside the stone. Opals also exhibit milky or smoky reflections from within the gem. Gems that are fibrous in structure show irregular interior reflections similar to those seen on watered or moiré silk. This optical property, which is called chatoyancy, is exhibited by several gems, notably the tiger's-eye and cat's-eye.

The appearance of a gem as seen by reflected light is another optical property of gemstones and is called lustre. The lustre of gems is characterized by the terms metallic, adamantine (like the lustre of a diamond), vitreous (like the lustre of glass), resinous, greasy, silky, pearly, or dull. Lustre is particularly important in the identification of gemstones in their uncut state.

III

Identification of Gems

A gem cannot always be identified by sight alone. It is therefore necessary to rely on measurement of the optical properties that can be determined without harming the stone in any way.

A gemmologist uses an instrument called a refractometer to measure the characteristic property of the stone, known as refractive index, which is its relative ability to refract light. In addition, an instrument called a polariscope is employed to determine whether a gem is doubly or singly refracting. Emeralds, rubies, sapphires, amethysts, and synthetic rubies and sapphires are all doubly refracting, whereas diamonds, spinels, synthetic spinels, garnets, and glass are singly refracting (see Crystals). A special dark-field illuminator with a binocular microscope is employed for examining the interior of a gemstone to determine whether it is of natural or artificial origin, and to search for inclusions characteristic of a given gemstone.

These tests are usually sufficient to identify the rather limited number of materials used as gemstones; occasionally, however, other instruments are required, including a dichroscope, which measures the property called dichroism, or a spectroscope to determine the characteristic absorption spectrum. Hardness, the test ordinarily associated with gem testing, is never used on cut stones by a gemmologist.

Another physical test that can be given to an unknown stone is the determination of its relative density, or density. For exact determinations various weighing devices are used, but rough approximations of the relative density of lighter stones can be made by means of a series of liquids of known relative density. If the stone will float in a liquid having a relative density of 4 and sink in a liquid with a relative density of 3, the relative density of the stone must lie between these limits and be approximately 3.5.

IV

Gem Materials

The accompanying table lists precious and semi-precious gem minerals with the names commonly applied to them. See separate articles on many of the gems mentioned.