Color in Gemstones – Idiochromatic, Allochromatic Coloration (12 July 2010)
The visual appeal and characteristics of a gemstone are determined by several factors including: brilliance (sparkle), color, fire (light dispersion), and luster (surface reflectiveness). A stone's brilliance, fire, and to a lesser extent luster, are influenced by the type of cut used.
The color of a gemstone is due to one or more of several key factors. A mineral can have its own inherent color due to its basic chemical makeup (idiochromatic), or it can start out as a colorless material and gain its characteristic color from trace impurities (allochromatic). A mineral can also display multi-colored characteristics from the scattering or dispersion of light (pseudochromatic) that is reflected from its surface, while having little or no inherent color.
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Optical and special effects in Gemstones (11 July 2010)
The visual appeal and characteristics of a gemstone are determined by several factors including: brilliance (sparkle), color, fire (light dispersion), and luster (surface reflectiveness). A stone's brilliance, fire, and to a lesser extent luster, are influenced by the type of cut used.
The faceting of a gemstone will affect how light will behave as it passes through the outside surface into the interior of the gem. Light can either be reflected off a surface affecting the luster of the stone, or pass through the surface into the mineral and be refracted, scattered, and dispersed as the light exits the stone. As a light beam passes through a gem it is bent or refracted before it exits the crystal. The light beam is also broken into its component parts (dispersion) causing the effect known as "fire". This dispersion widens the beam to the point that the observer can see the full visible spectrum of the beam from red to violet, simulating a rainbow effect. As the stone is moved, the refraction and reflection points of the facets change showing the stone's scintillation or "play of color."
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Diamonds – 57 facet round brilliant (01 July 2010)
The original round brilliant-cut was developed by Marcel Tolkowsky in 1919. The modern round brilliant consists of 58 facets (or 57 if the culet is excluded), ordinarily today cut in two pyramids placed base to base: 33 on the crown (the top half above the middle or girdle of the stone), truncated comparatively near its base by the table, and 25 on the pavilion (the lower half below the girdle), which has only the apex cut off to form the culet, around which 8 extra facets are sometimes added. In recent decades, most girdles are faceted. Many girdles have 32, 64, 80, or 96 facets; these facets are not counted in the total. While the facet count is standard, the actual proportions (crown height and angle, pavilion depth, etc.) are not universally agreed upon.
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