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Carat The weight used in the gem trade since antiquity. The name is derived from the seed (kuara) of the African Coraltree or from the kernel (Greek -kertion) of the Carob bean. Since 1907 Europe, as well as America, has adopted the metric carat of 200mg or 0.2g. Therefore weights given for famous old diamonds often vary because local carats and not metric carats were used. The carat is subdivided into fractions (l/10ct) or decimals (1.25ct) up to two decimal places. Small diamonds are weighted in "points" = l/100cts (=0.0lcts). The table below illustrates diameter and corresponding carat weight for diamonds cut in the modern brilliant cut (p. 81). Gems with different specific gravity and different cuts obviously have different diameters vintage engagement rings. The carat weight of gems is not to be confused with the carat used by the goldsmith. In the case of gold the carat is no weight measure but a designation of quality. The higher the caratage, the higher the content of gold in the piece of jewelry. The weight can be variable.

Gram Weight used in gem trade for less precious stones and for rough stones (for instance quartz).

Grain (Latin - granum) Weight measure for pearls. Corresponds to 0.05 grams = 0.25 or \ carat. Increasingly superceded by carat weights. The old Japanese weight of Momme (=3.75 grams= 18.75 carats) is hardly used any more in Europe.

Prices in the Gem Trade The price quoted is always the carat price. This has to be multiplied by the actual weight to obtain the price of the particular stone. The price quoted to the final buyer is often the total price. With diamonds the price per carat shop for tacori engagement rings increases progressively with the size of the gem in very small steps, 10 points with more expensive stones. If, for instance, a one carat stone costs £750/$1275, a two carat stone (of the same quality) does not cost £1500/$2550 per carat, but maybe £2500/14250 or more. Colored stones have fixed carat prices for much bigger size groups.

Of all the various properties of a gemstone the optical characteristics are of unsurpassed importance diamond solitare earrings. They produce color and luster, fire and lumines¬cence, play of light and schiller (iridescence). In the examination of gems there is more and more concentration on the optical effects.

Color

Color is the most important characteristic of gems. In the case of most stones, it is not diagnostic in identification, because many have the same color and numerous stones occur in many colors. Color is produced by light; light is electromagnetic vibration at certain wave lengths. The human eye can only perceive wave lengths between 4000A to 7000A (p. 36). This visible light falls into six parts, each of a particular color (spectral colors: red, orange, yellow, green, blue, violet). The mixture of all these produces white light. If, however, a certain wavelength is absorbed, the remaining mixture produces a color, but not white. If all wave lengths pass through the stone, it appears colorless; if all light is absorbed the stone appears black. If all wave lengths are absorbed to the same degree, the stone is dull white or gray. If, however, only certain wavelengths are absorbed, the stone will have the color resulting from the remaining spectral mixture.

In the case of gemstones, the metals, mainly chrome, iron, cobalt, copper, manganese, nickel and vanadium, absorb certain wavelengths of white light and so cause coloration. These substances sometimes appear in such small quantities in the stone that they are not indicated in the chemical formula. In the case of zircon and smoky quartz, no foreign substance is responsible for the color; it is caused by the deformation of the internal structure (lattice) and is produced by short wave rays from the atmosphere (ultra-violet rays) resulting in a selective absorption.

The distance the light ray travels through the stone can also influence absorption and thus color. The cutter must therefore use this fact to his advantage. Light colored stones are made thicker and/or are given such an arrangement of facets that the absorption path lengthens and the color deepens, while materials with colors that are too dark are cut thinly. The dark red almandine garnet (p. 104) is therefore often hollowed out on the underside design your own wedding ring. Artificial light has an influence on the color of a gemstone as it is differently composed from daylight. Artificial light has an unfavorable effect on some stones (for instance sapphire) and a favorable effect on others (emerald and ruby). The most obvious change occurs in alexandrite (p. 98) which is green in daylight and red in artificial light.

Although color is of great importance in gems, with the exception of diamonds, no practical method of objective color determination is known. Color comparison charts are poor substitutes because there is much room for subjective consideration. The measuring methods used in science for color determination are too complicated and expensive for the trade.

Color of streak

The color appearance of gems, even in the same group, can vary greatly. For instance beryl can have all the colors of the spectrum, but can also be color¬less. This colorlessness is the true color; it is called "inherent color". All others are produced by coloring substances. The inherent color, as it is constant, can help to identify a stone. The color can be seen by streaking the mineral on a rough porcelain plate, called a "streak plate", because the finely ground powder has the same effect as thin transparent platelets. Steely hematite for instance has a streak color (called streak) which is red. Brass-colored pyrite's streak is black and blue sodalite's is white. In the case of a very hard mineral, it is advisable first to remove a little powder with a steel file and then rub it on the streak plate. These methods of determination are of special interest to collectors. Because of the danger of damaging them, cut gems should not be tested for streak. See Table of Streaks, p. 30.

Color change

The color of some gems is altered by time. Amethyst, rose quartz and kunzite can become paler when exposed to direct sunlight. Generally color changes effected by natural causes are not common. Much more frequently man uses scientific methods to enhance the color of certain gemstones.

Best known is the heat treatment of amethyst. At several hundred degrees, the original violet stone becomes light yellow, red-brown, green or milky white. Most citrines on sale and all prasiolites are amethysts treated in this way. Less attractive colors can be changed to more desirable hues by heating. Greenish aquamarines are heated to a sea-blue color; tourmalines which are too dark can be lightened; and blue tourmalines can be turned green. Diamond-like and aquamarine-colored zircons are produced by heating the red-brown hyacinth variety.

Colors can also be improved by radium and x-ray treatment and, more recently, by bombardment of cheap engagement rings with electrons in an accelerator or with neutrons in an atomic reactor. The resulting colors are sometimes so close to nature that they cannot be detected by the eye; complicated tests are required to unmask them. Some of these resulting colors are not permanent; the stones can become pale, change color or become spotty. In the case of porous gems, such as lapis lazuli, turquoise, pearls and agate, colors are improved by the addition of a pigment. Stone dyeing is a very ancient practice. See coloring of agate, p. 136.

All gems with artificial color changes - with the exception of heat-treated stones and dyed agates - have to be marked as such when offered for sale.

Refractive Index

Most of us, when children, noticed that when a stick was partially immersed in water at a slant, it appeared to "break" at water level. The lower part of the stick appeared to be at a different angle from the upper part. What we observed was caused by the refraction of light. It always occurs, when a ray of light leaves one medium (for instance air) and enters obliquely into another (for instance a gem crystal), at the interface between the two materials.

The amount of refraction of crystals is constant in the various types of gems. It can therefore be used in the identification of the type of stone. The amount of the refraction is called the refractive index and is defined as the proportional relation between the speed of light in air to that in the stone. A decrease in the velocity of light in the stone causes a deviation of the light rays.

The speed of light in air is 2.4 times faster than the speed of light in diamond. The refractive indices of gems are between 1.2 and 2.6. They vary somewhat with color and occurrence. Doubly refractive gems (see p. 34) have two refractive indices. See Table of refractive indices, p. 32.

The refractive index is measured with a refractometer. The values can be read directly from a scale. However testing is only possible up to a value of 1.86 on a common instrument and only stones with a flat face or facet are suitable, although the expert can find the approximate value of cabochons using his own knowledge. It is fairly easy to measure the refractive index by the immersion method. The gem is viewed after immersing it in a liquid whose refractive index is known. The refractive index of the stone can then be judged according to its brightness and sharpness of its outline or contours.

Double Refraction

In all gemstones, except opals, glasses and those belonging to the Isometric (cubic) system, the ray of light enters the crystal and is divided into two rays. This is called double refraction. It can be most easily observed in the case of Iceland spar. It is also easily seen in zircon, sphene and peridot; when looking through the top, one can see the doubling of the edges of the lower facets with the naked eye. In the case of synthetic rutile, the double refraction is so strong that the stone can give a blurred impression. It is up to the lapidary to work the stone in such a way that the double refraction does not appear disturbing. In the case of most gems, double refraction is small and cannot be detected without instruments. Therefore double refraction can help to identify gems. It is expressed as the difference between the highest and lowest refractive indices. The expert also differentiates between positive and negative "optical character". (For data on double refraction, see the table on p. 32.)

Dispersion

White light is split not only when passing through a crystal, but also when it fans out into its spectral colors. The deviation is dependent on the density and on wave lengths of the light. Because the individual spectral colors have different wave lengths, they are also bent differently. (See illustrations p. 36). For instance, in the case of diamond, the refractive index of the red (at a wave length of 6870 A) is 2.407, of the yellow (5890 A) is 2.417, of the green (5270 A) is 2.427 and of the violet (3970 A) is 2.465. This division of white light into colors of the rainbow is called dispersion.

Color dispersion is particularly large in diamonds where it produces a beautiful play of color, the so-called "fire". Only colorless gems have good dispersion. Natural as well as synthetic gemstones with high dispersion (for instance strontium titanate, rutile, sphalerite, sphene, zircon) are used as substitutes for diamond The dispersion of a stone is expressed in figures as the difference between the red and violet refractive indices. As the color usually comprises a wide spectrum, it is common to measure the Fraunhofer B-line for the red and the G-line for the violet.

Absorption Spectra

The absorption spectrum of a gem is one of the most important aids in identifying it. The absorption spectrum of a stone are the bands of spectral colors of light which have passed through the gem (ill. p. 39). As we know, certain wavelengths (color bands) of the light are absorbed (see p. 27) and the color of the gem is formed from the mixture of the remaining parts of the original white light. The human eye cannot recognize all minute color differences cheap diamond engagement rings . Red tourmaline, red garnet or even red-colored glass can appear deceivingly like the desirable red ruby. However the absorption spectrum unmasks without any doubt the stone or glass used to imitate the ruby. Most gems have a very characteristic, and even unique, absorption spectrum which is revealed in black vertical lines or broad bands.

The great advantage of the testing method is the ease with which one can differentiate between gems of similar specific gravity and refractive index. One can also use this method for testing rough stones, cabochons and even set stones. An important area where absorption spectroscopy is applied is in differentiating between natural stones, synthetic stones and imitations.

Best results are obtained from strongly colored transparent gemstones. The observation of the absorption spectrum of an opaque stone is only possible when a thin slice of the stone is prepared which may transmit light (as in the case of hematite). Otherwise a translucent edge must be presented, or light must be reflected from the surface.

The testing instrument is the spectroscope, with the help of which one can determine the wavelength of the absorbed light. The wavelength is measured in Angstrom units (lA - 1 ten millionth of a millimeter), and more recently also in nanometers (1 NM - 1 millionth millimeter). Because the absorption lines or bands are not always of equal strength, it is usual to note such measured differences. Strong absorption lines are underlined, for instance 6535; weak lines are bracketed, for instance (4327).

Transparency

A factor in the evaluation of most gemstones is their transparency. Inclusions of foreign matter and air bubbles or fissures in the interior of the crystal affect the transparency. The path of light through the crystal can also be impaired by strong absofption in the crystal. Grainy, stalky or fibrous aggregates (such as chalcedony, lapis lazuli, turquoise) are opaque because the rays of light are repeatedly refracted or reflected by the many tiny faces until finally they are completely reflected or absorbed. Where the light is only weakened by its passage through a stone, it is said to have translucency.

Luster

The luster of a gem is caused by reflection, i.e. the reflecting of part of the incident light back from the surface. It is dependent on the refractive index and the nature of the surface, but not on the color. The higher the refraction, the higher the luster. Most desirable luster is adamantine, the most common is vitreous. Comparatively rare are the fatty, metallic, pearly, silky and waxy lusters. Stones with no luster are described as dull.

In everyday language, those light effects which are caused by total reflection are also considered as luster. The lower facets of the gem act as a mirror and reflect the entering light more or less completely, so strengthening the lustrous appearance. The phenomenon on the surface of the stone is called brilliance. The ideal complete refraction is found in the diamond cut which thus reaches the highest brilliance.

Pleochroism

Some gems appear to have different colors or depth of color when viewed in different directions. This is caused by the differing absorption of light of doubly refractive crystals. Where two main colors can be observed (only in the tetragonal, hexagonal and trigonal crystal systems), one speaks of dichroism; where three colors can be seen (only in the orthorhombic, mono-clinic and triclinic crystal systems), of trichroism or pleochroism. The latter is a collective description for both phenomena. Amorphous gems and those in the isometric (cubic) system show no pleochroism.

Pleochroism can be weak, definite or strong. It must be taken into consideration antique style engagement rings  when cutting in order to avoid poor colors, or shades that are too dark or too light.

Light and color effects

Many gems show striated light effects or color effects which do not relate to their color and are not caused by impurities or their chemical composition. These effects are caused by reflection, interference and refraction.

Chatoyancy (cat's eye effect) An effect which resembles the slit eye of a cat (French chat=cat, oeil~eye): this is caused by the reflection of light by parallel fibers, needles or channels. This phenomenon is most effective when the stone is cut en cabochon in such a way that the base is parallel to the fibers. When the gem is rotated, the cat's eye glides over the surface. The most precious cat's eye is that of chrysoberyl (p. 198). The effect can be found in many gemstones, especially well known are quartz cat's eye, hawk's eye and tiger's eye (p. 124). If one talks simply of cat's eye, one refers to a chrysoberyl cat's eye. All other cat's eye must have an additional designation.

Aslerism This is the effect of light rays forming a star (Greek aster=star); the rays meet in one point and enclose definite angles (depending on the symmetry of the stone). It is formed like a cat's eye, but the reflecting fibers lie in various directions. Ruby (p. 82) and sapphire (p. 86) cabochons can show effective six-rayed stars. There are also four-rayed stars and, rarely, twelve-rayed stars. If a piece of rose quartz has been cut as a sphere, the rays move in circles over the whole surface; where included needles are partially destroyed, stunted stars, part circles or light clusters are formed. Asterism also occurs in synthetic gems.

Adularescence Moonstone, being a variety of adularia (see name, p. 164) shows a blue-whitish opalescence which glides over the surface when the stone is cut en cabochon. Interference phenomena of the layered structure are the cause of this effect.

Aventurization Colorful play of glittering reflections of small, leaf-like inclusions on an opaque background. The inclusions are hematite or goethite in the case of aventurine feldspar (p. 166); fuchsite or hematite in aventurine quartz (p. 122); and copper scrapings in imitation aventurine glass.

Iridescence Play of color of some gems caused by dispersion of light in cracks and flaws resulting in the colors of the rainbow (Greek »m=rainbow). Commercially this effect is created by artificially producing cracks in rock crystal.

Labradorescence Play of color in metallic hues, especially in labradorite (hence the name) and spectrolite (p. 166). Blue and green effects are often found, but the whole spectrum is most desirable. The cause of the schillers is most probably interference phenomena of twinned layering.

Opalescence Milky blue or pearly appearance of common opal (p. 152) caused by reflection of short wave, mainly blue, light. It should not be confused with opalization.

Opalization (often called iridescence) Play of color of the opal (hence the name, p. 150) which changes with the angle of observation. As recently as the 1960s, this effect was explained as the light refraction of the smallest layered structure. The electron-microscope shows the real cause at a magnification of 20,000x: small spheres of the mineral cristobalite included in a silica gel cause the reflection interference phenomena. The diameter of these spheres is one ten-thousandth of a millimeter. The term should not be confused with opalescence (p. 44).

Silk Reflection of parallel fibrous inclusions or canals causes a silk-like appearance. Not desirable in faceted rubies and sapphires. Where the included needles are too numerous, the stone becomes opaque and, when cut accord¬ingly, can show a cat's eye.

Luminescence

Luminescence (Latin - light) is a collective definition for the emission of visible light under the influence of certain rays, as well as by some physical or chemical reaction, but not including pure heat radiation. The most important of these phenomena for the testing of gems is the luminescence under ultra¬violet light which is called fluorescence. The name fluorescence is derived from the mineral fluorite, which was the substance in which this light pheno¬menon was first observed. When the substance continues to give out light after irradiation has ceased, the effect is called phosphorescence (named after the well-known light property of phosphorus).

The cause of fluorescence in gems is the incorporation of small metal impurities which often also affect the color, such as chromium, manganese, cobalt and nickel, as well as molybdate, tungsten and some uranium com¬pounds. As various types of gems can contain different trace elements, not all stones within a group necessarily show the same fluorescent colors. Some deposits produce very characteristic fluorescent colors. Iron, if present even in the smallest quantities, prevents any fluorescence.

Gems are tested in ultra-violet light with long waves (4000-3150A) and short waves (2800-2000A). This is because some gems react only to one of these wave lengths. The intermediary wave lengths (3150-2800A) are of no importance in gem testing. It is usual to designate long waves as 3650 A and short waves as 2537A. Fluorescence can help to identify gems and is particu¬larly useful in recognizing synthetic stones. The strength of fluorescence can vary, and the color can be one of many hues.

Luminescence caused by x-rays can help to differentiate between real and cultured pearls. The mother-of-pearl of the salt-water pearl oyster does not luminesce while that of the fresh-water pearl mussel gives off a strong light. As the inserted nucleus of a cultured pearl has been taken from a piece of fresh-water mother-of-pearl, the cultured pearl shows a luminescence which the real pearls do not have. See Table of Fluorescence of Gems, p. 46.