Color of diamonds

Processes involving defect energy levels are responsible for coloration of diamonds containing races of nitrogen or boron impurities. Diamond has a band gap of about 8.65 x 10-19 J (5.4 eV), which is too large to absorb visible light and... 

The color of diamond due to nitrogen impurities has been described in Section 9.6.3 It has been found that nitrogen impurities that are located next to a carbon vacancy in diamond thin films endow the solid with quite new properties, somewhat similar to the properties of a solid containing FLi centers compared with ordinary F centers. The diamond structure is built up of carbon atoms each surrounded by four... 

Determination of tint is the essential and the most intricate aspect in diamonds evaluation and may be useful for luminescence interpretation. The coloration of diamond is a reflection of its complex structural pecuharity. To describe a slight shade of color, one has to use a lot of physics, crystallography and analytical tools. The following types of diamond coloration are generally distinguished. 

Several gemstone species occur in various colors, depending on the presence of impurities or irradiation-induced color centers. Examples are the beryl, comndum, and quart2 families. Quart2 has poor optical properties (RI = 1.55, DISP = 0.013), but becomes of gemological interest when it exhibits attractive colors. Any material can have its color modified by the addition of various impurities synthetic mby, sapphires, and spinel are produced commercially in over 100 colors (2). Synthetic cubic 2irconia has been made in essentially all colors of the spectmm (11), but only the colorless diamond imitation is produced commercially in any quantity. 

With the exception of diamond coloring and the turning of topa2 blue, the source of the irradiation is immaterial. Gamma rays are the preferred source because of uniformity of coloration and the absence of heating and induced radioactivity. The most commonly seen gemstones enhanced by irradiation are summari2ed in Table 4. 

Boron is electron deficient relative to carbon. Therefore, small amounts of boron, replacing carbon in a diamond lattice, causes electron holes. As electrons move to fill these lattice vacancies, infrared light is absorbed causing the blue color of the Hope diamond and other blue diamonds. 

Diamonds are the only gemstone whose colorlessness enhances their value. However, the rare, rich, natural coloring of fancy color diamonds commands the highest prices. The Hope Diamond possesses exceptional blue coloring and is undoubtedly the most celebrated diamond in the world. What is the origin of its intensely prized blue hue ... 

Throughout history civilization has treasured the rarity and beauty of fancy colored diamonds. The stunning diamond from India known as the Hope Diamond, once a part of many royal inventories, is now the premier attraction of the Smithsonian Institution (see color Fig. 4.3.1). While the size of the diamond at 45.52 carats has certainly contributed to the public s interest in the gem, the intense blue-violet color of the stone is generally considered to be its most captivating feature. First described in the mid 1600s by the French merchant traveller Jean Baptiste Tavernier as un beau violet (a beautiful violet), the gem also acquired the title Blue Diamond of the Crown or the Royal French Blue when in possession of King Louis XIV of France. The blue color is attributed to trace amounts of boron in the carbon matrix of the stone. Substitution of carbon atoms by nitrogen leads to yellow diamonds, such as the famous canary yellow 128.51-carat Tiffany diamond. 

Cut the resulting blocks with a diamond knife, and mount sections on nickel grids. It is often advisable to cut the sections slightly thicker than normal such that the interference color of the sections is light gold (see Note 14). 

The diamond is found in natural deposits in many parts of the world. Also, it can be synthesized from graphite or other carbonaceous materials. Graphite can be converted to diamond under high temperatures (about 1,400°C) and very high pressure (in the range 4,000-5,000 atm) in the presence of a metal catalyst such as iron or nickel. Presence of trace impurities can impart different coloration to diamonds. For example, introducing trace boron or nitrogen causes blue or yellow coloration. 

Diamonds are the hardest natural substance, but they can cleave easily. They occur in every color, but shades of yellow and brown are most common. Today s faceted diamonds are cut and polished with tools embedded with tiny diamond chips or dust. Before the advent of modern methods, diamonds were used in their natural state, or roughly shaped and polished by hand. The hardness of this mineral led many early jewelers to do a minimum of work before setting diamonds in jewelry or regalia. Common imitations of diamond are colorless spinel, sapphire, zircon, topaz, quartz, and many synthetics. 

Colorless zircon has been used as an imitation of diamond, since it has a very high refractive index. Zircon gems are doubly refractive, which can give them a cloudy appearance. Looking through the top of a faceted zircon, the back facet edges will appear double. Zircons come in many colors, and this gem is often heat-treated to change or enhance the color. 

---