The color of most natural black diamonds is caused by graphite contained in the crystal. Under the microscope, these graphites are usually tiny flakes, which are evenly distributed in the crystals of diamonds. Sometimes there are large black graphite inclusions, which can even be observed directly with the naked eye. Because graphite has a very high absorption rate of visible light, it absorbs almost all the incident light. If there is more graphite, the diamonds will all be black. Although graphite has a strong absorption of visible light, because of the high refractive index of diamonds and high reflectivity of surface light, black diamonds that have been cut and polished, like diamonds of other colors, have shiny spots on their surfaces, which is very attractive and equally attractive.
When the graphite content in natural diamonds is very low, diamonds are gray. Because the absorption rate of graphite to visible light is extremely high, low graphite content is enough to produce gray. The gray color of natural diamonds is definitely not caused by the so-called mixture of black graphite and white diffuse reflector, but only because the graphite content is very low. Opaque gray diamonds can hardly be found in the jewelry market. An opaque gray diamond can be obtained by cutting the gray twin diamond shown in figure 1-4 in the first chapter.
When a diamond has N3, H3, N-V and GR 1 color centers at the same time, if these color centers form a basically flat spectral absorption curve in the visible wavelength range, its color may be gray with the same color in different spectra. The minimum requirement of this metamerism gray spectrum is that it has at least three intersections with the horizontal curve representing the corresponding gray in the visible wavelength range. Obviously, the more intersections there are, the more likely it is to appear gray. This kind of grey diamond has not been reported so far.
The color of black diamonds produced in Siberia is caused by a large number of magnetite inclusions, and the color of black-gray diamonds is mainly caused by hematite and pure iron inclusions. Magnetic black diamonds are generally natural diamonds produced in Siberia. Using the iron mineral inclusions of black diamonds, it can be identified that the origin of black diamonds is Siberia, as long as the black diamonds attracted by magnets are likely to come from Siberia.
Due to the increasing demand for black diamonds, artificially processed black diamonds also appear in the diamond market. The color causes of synthetic black diamonds are not exactly the same as those of natural black diamonds. The color of the former is produced by the intense radiation treatment of damaged lattice and holes, and the radiation source used is high-energy particles, mainly high-energy neutron beams. High-energy neutron radiation can produce a lot of lattice damage and holes in diamond at high temperature, and the scope of lattice damage is much larger than holes, which is equivalent to reducing diamond crystals into carbon black with disordered atomic arrangement. Carbon black has a strong non-selective absorption of light, which is the main reason for the black color produced by artificial treatment of black diamonds. At the same time, the GR 1 color center caused by a large number of holes also produced some absorption. The primary graphite, a large amount of carbon black and strong GR 1 color center in the diamond make the diamond appear black. Under strong light, the edge of the artificially treated black diamond appears dark green; If the irradiated black diamond is sliced, the color of the transmitted light is also dark green.
Figure 3- 14 A colored black natural diamond collected by the author.
(Ada Photography/Ada Collection)
According to the author's understanding, when black diamond is irradiated, the diamond is in a vacuum high temperature state (about 700℃ or higher). The lattice thermal vibration of diamond is intensified at high temperature. Under the bombardment of high-energy neutrons, the crystal is easily destroyed and turned into carbon black, which makes the color of the diamond black. At present, most of the artificially colored black diamonds may have undergone this discoloration treatment. The original diamond used is low-grade clarity or industrial black-gray diamond, which is sold as black in the market, greatly improving the price and people's love.
Figure 3- 14 is a typical black natural diamond collected by the author. The original stone of this black diamond should be industrial-grade gray-black natural diamond, and the crystal contains a lot of black graphite. After intense radiation treatment, a large number of carbon black and holes were produced, which became "pure" black. Under strong light, dark green transmitted light can be observed along the waist.
The most famous black diamond is the 67.50-carat rlov diamond. According to legend, this diamond originally belonged to a temple in India and weighed 195ct. Russian Duchess Olav once owned this diamond and named it after her. Olaf diamonds have been exhibited in many parts of the world and sold by auction many times. On June, 2006, 65438+1October1day, it was sold at Christie's "M agnificent Jew els" auction for $352000, which was the highest price for black diamonds.