Gemstone Basics cont'd
Classic Crafts
The weight used in gem trade is carat. The carat is subdivided into fractions or decimals up to 2 decimal places. Small diamonds are weighed in points, which are 1/100 of a carat. The price of a gemstone is usually indicated in the gem trade as per carat. By calculating the actual weight, one receives the price per piece. This is not the same as karat used by the goldsmith. Gram is the weight measure used in the trade for less precious gemstones and especially for rough stones. Grain used to be the weight measure for pearls. Today it is increasingly substituted by the use of carat.
Color is the most important characteristic of gems. Many stones have the same color and many stones come in many colors. Color is produced by light. Light is an electromagnetic vibration at certain wavelengths. The mixture of the multiple colors produces white light. Certain wavelengths are absorbed out of the spectrum, the remaining mixture produces a certain color, but not white. If all the wavelengths pass through the stone, it appears colorless. If all light is absorbed, the stone appears black. If all wavelengths are absorbed to the same degree, the stone is dull white or gray. Refraction is caused when a ray of light leaves one medium (ex. air) and enters into another (ex. water) at the interface between the two. The amount of refraction in the crystals is constant for each specific gemstone. This makes it able to be used in the identification of the type of stone. The amount of the refraction is called the refractive index. A tool called a refractometer can measure light refraction. Only stones with a flat face or facet are suitable to measured with it though.
Dispersion is caused when one can observe flashes of color in a colorless and cut gemstone. The dispersion is different from one gemstone to another. Facets can enhance the dispersion. Colorful gemstones tend to mask the dispersion effect. Dispersion can be especially high in diamonds. the dispersion can be measured with a refractometer and other special devices. The dispersion of a stone is expressed in figures as the difference between the red and violet refractive indices. It is important to remember that only transparent stones can show any dispersion at all. A spectroscope can determine the wavelength of the absorbed light. The wavelength is measured in nonometers, symbol nm. The former measurement Angstrom, symbol A is still widely used in gemological literature.
Transparency is a factor in evaluating most gemstones. Inclusions of foreign matter or fissures in the interior of the crystal affect the transparency or "clarity". Grainy, stalky, or fibrous aggregates are opaque because the rays of light are repeatedly refracted or reflected by the many tine 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 of a gem is caused by external reflection. The reflecting of part of the incident light back from the surface 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. The highest luster seen in transparent gemstones is a diamond like look. Stones with no luster are described as dull. The light effects which are caused by total reflection are considered as brilliance or light return. In transparent faceted gemstones the lower facets act as a mirror and reflect the entering light more or less completely, thus creating the brilliance. The ideal complete internal reflection is found in the diamond cut, which thus reaches the highest brilliance. Some gems appear to have different colors or depth of color when viewed in different directions. This caused by the differing absorption of light rays in doubly refractive crystals. Where 2 main colors can be seen is called dichroism. When 3 colors can be seen it is trichroism or pleoochroism. Opaque stones and most that are translucent usually do not show pleochroism. Luminescence is a term for the emission of visible light under the influence of certain rays and by physical or chemical reaction. The most important is to test the gem for luminescence under ultraviolet light called fluorescence. The causes of fluorescence are certain interference factors such as impurities or flaws in the crystal lattice.