Natural red diamonds are very rare. At present, the largest red diamond in the world-Moussaieff red diamond with bright triangular cut weighs 5. 1 1ct. It is reported that this red diamond was discovered by a Brazilian farmer in the mid-1990s. The original stone weighs about 1 1ct. Museyev red diamonds are on display at the Smithsonian Museum in Washington, D.C., USA. Another very famous red diamond is a 0.95-carat purple diamond, which sells for $880,000. Generally mistaken for red, the real color of this diamond is purplish red, not pure red tone. It was the 0.95-carat red diamond that aroused great interest in colored diamond from the jewelry industry and the public. According to literature and auction records, there are only four real red diamonds in the world so far, and there are more than a dozen other purple diamonds. Because the color evaluation of red diamonds is very harsh, so far, there are no natural diamonds with red tones such as crimson, brownish red or orange red, and there is no colored diamond with red tones such as red orange, red purple and reddish brown.

The color of natural red diamond may be caused by plastic deformation and nitrogen doping. Plastic deformation may produce pink with purple hue with high saturation. If the plastically deformed diamond contains a certain amount of nitrogen, both discrete nitrogen and polymeric nitrogen will absorb short-wave blue-violet light. Because the short-wave blue-violet light is partially absorbed by nitrogen atoms, the middle-band green-yellow-orange is partially absorbed by plastic deformation, and red becomes the main color, so the diamond will appear red. Although red diamonds can be produced theoretically, the probability of producing them in nature is almost zero. The main reason is that the selective absorption of plastic deformation is weak, and the colors produced are almost all pink and brown purple tones with low saturation, and it is almost impossible to produce red.

The 0.95-carat red diamond, which was sold for $880,000, can be observed under the microscope as a strip structure formed by obvious plastic deformation. Its absorption spectrum has a sharp absorption peak at 4 15nm, a weak absorption band at 495 ~ 5 10 nm and an absorption band at 530 ~ 590 nm. The absorption peak at 4 15nm is the zero phonon line of N 3 color center. The weak absorption band between 495 and 5 10 nm may be composed of 503.2nm H 3 color center and 496nm H 4 color center. The absorption band between 550nm is a plastic deformation absorption peak centered at 550nm. The color of this red diamond may be caused by plastic deformation, with N3 color center, H3 color center and H4 color center. Among them, plastic deformation mainly absorbs middle-band visible light, N3 color center, H3 color center and H4 color center mainly absorb short-wave visible light, and the remaining long-wave visible light makes the diamond appear red. The absorption intensity of the N3 color center, H3 color center and H4 color center of this red diamond is not very strong, so there is still a small amount of short-wave visible light in the remaining visible light, thus showing a purple color.

The ultraviolet fluorescence of this 0.95-carat red diamond shows blue and pink bands. Blue fluorescence should be caused by N 3 color center radiation, and pink fluorescence should be mainly caused by plastic deformation color center radiation. Its fluorescence is weak, which may be due to the high content of type A nitrogen polymer, which inhibits the fluorescence radiation. The reason for the banded distribution of ultraviolet fluorescence is the banded distribution of plastic deformation. The banded areas with plastic deformation emit pink fluorescence, and the rest areas emit blue fluorescence.

In recent decades, synthetic diamond technology has developed rapidly. Artificial diamonds were treated to get "red" diamonds by accident. Up to now, almost all these artificial red diamonds are reddish brown or reddish brown. After cutting and polishing, they show a red flashing area on a brown background, so they are called red artificial diamonds. The color tone of the synthetic red diamond shown in Figure 2- 13 is purplish red under sunlight and standard D65 light source, which is the color tone of a typical N-V color center synthetic red diamond. According to the author's measurement and research, it is also confirmed that up to now, all synthetic red diamonds are purplish red, not pure red, mainly because they have strong N-V color centers and can produce purplish red tones.

When the nitrogen content of synthetic diamond is high, it will produce deep yellow or even olive color. Yellow is because free nitrogen atoms absorb visible light with wavelength less than 56 1nm, and the absorption intensity of light increases with the decrease of wavelength. This kind of yellow diamond will produce crystal damage holes after radiation treatment; After high temperature and high pressure treatment, holes combine with discrete nitrogen atoms to form new N-V, H3 and H4 color centers. N-V color center will absorb the middle band of visible light, N3 color center, H3 color center and H4 color center will absorb short-wave visible light, and finally only long-wave red light will not be absorbed, so the treated synthetic diamond appears red. The N-V color center of this red synthetic diamond has strong absorption, which makes the absorption intensity of mid-band visible light greater than that of short-wave visible light, resulting in the color tone of the diamond being purplish red instead of pure red.

When the yellow synthetic diamond with high saturation mainly produces N-V color centers after radiation and heat treatment, little or no H3 and H4 color centers are produced, and the diamond also appears purplish red. Because of the low temperature and pressure of heat treatment, the polymer of nitrogen can't or can't be formed quickly, so the color centers of H3 and H4 can't be formed further. Therefore, the color of synthetic diamonds is more purplish red.

The surface color of diamonds is related to cutting, and the surface color of red synthetic diamonds is no exception. The best color can be obtained by changing the cutting mode and cutting angle. The ideal bright circle can get the best color, and the cut pattern of red synthetic diamond is generally an ideal bright circle, regardless of retaining the maximum weight cut pattern.

Little-known natural red diamond. 1987 10 The 0.95-carat purple-red red diamond auctioned in new york was sold for $880,000, creating the highest carat price in diamond history. According to the records, this red diamond was bought by a collector in Montana, USA in 1956 for 13500 USD. The origin is unknown, it may be Brazil. The price of this red diamond rose to $880,000 in a short time. It can be seen that colored diamond has great collection value and appreciation space. The carat price of Museyev diamond should far exceed that of 0.95 carat red diamond.

Natural red diamonds are too expensive for the public to appreciate, but processed red synthetic diamonds are gradually increasing in the international jewelry market. According to the author's observation, infrared-visible-ultraviolet spectrum measurement and research on the purple-red synthetic diamond and other red synthetic diamonds shown in Figure 2- 13, they all belong to IB type containing free nitrogen and have N-V color centers after radiation and heat treatment. The diamond shown in Figure 2- 13 is a typical IB red synthetic diamond after radiation and heat treatment, and the colors of other red synthetic diamonds are similar to their colors.

Compared with natural red diamonds, synthetic red diamonds are quite cheap, but as daily ornaments, synthetic red diamonds are the best choice.