Color of gem minerals

1. Definition

Color is the result of human eye color stimulated by light and translated by brain. The perception of color has experienced a complicated process, which involves optics, photochemistry, visual physiology, visual psychology and so on.

Only visible light waves can be felt by human eyes and produce vision. The wavelength range of visible light is between 380 and 760 nm, and different wavelengths of radiation cause different color perception for human eyes. The colors corresponding to each wavelength are divided as follows:

Table 4- 1- 1 Wavelengths and corresponding colors

The above division is only rough. In fact, one color gradually changes to another color, and the color changes unevenly with the wavelength, as shown in Figure 4- 1- 1, and the spectral color in the visible region is uneven. From the ultraviolet region to the infrared region, its wavelength increases and its energy decreases.

Some light sources themselves can make people feel the color, which is called light source color, and its color corresponds to a certain wavelength or energy. To see the color of matter, human eyes must meet three conditions: ① external illumination; (2) Interaction between external radiation and gems; (3) The human eye receives color stimuli and the brain interprets them. For these three situations, we explain as follows. Obviously, if there is no light, such as in the dark, you can't feel the color of the gem. As a form of energy, light interacts with precious stones as soon as it touches them. Different gems have different compositions and structures, so the interaction with light will produce different results, which will lead to different colors. When people feel color, their eyes should first accept color stimulation. For example, blind people can't see colors. However, the brain's translation of colors can't be completely explained by physical stimuli. For example, for black coal in the sun and white chalk in the shadow, the measured results show that the former reflects more light than the latter. At this time, the role played by the human brain may be related to memory, so it still feels that the former is black and the latter is white.

Fig. 4- 1- 1 Color change of visible area

2. Color description

Colors can generally be divided into colored and colorless, and their basic names are as follows (Table 4- 1-2). For color series, you can also add modifiers to express lightness and saturation before its basic name, such as light green, dark blue, true red and so on.

Table 4- 1-2 Basic names of colors

Second, selective absorption of light.

As mentioned above, there are three conditions for human eyes to feel color. One is the interaction between external illumination and objects. In gemmology, we often talk about the absorption of light by gems. When the light passes through a gem, the intensity of the light decreases with the deepening of the light, which is that the gem absorbs the light. Selective absorption of light means that a gem absorbs some wavelengths of light better than others. The color of gemstones is the result of selective absorption of visible light, which shows that gemstones have absorption spectra at some visible wavelengths.

The selective absorption of light by gemstones is related to its composition and structure, and the main theories involved will be introduced in the next part.

Figure 4- 1-2 Transmission of Transparent Objects

As shown in Figure 4- 1-2, most gems are transparent and some light is reflected from the surface, as shown in Figure A; After other light is selectively absorbed by the gemstone, the remaining light is emitted from the back, as shown in the figure, transmitted light B; Finally, the colors of all transmitted light are expressed as the colors of transparent gemstones. For example, the green glass imitation gem completely absorbs the light at both ends of the red and blue spectrum, and finally only the green light passes through, so it appears green. Of course, the selective absorption of most gemstones is more complicated, but the color of gemstones is still the color of transmitted light after selectively absorbing visible light. By testing the transmitted light, we can know the selective absorption of the gemstone and make a spectral distribution diagram to characterize the color of the gemstone.

Third, the principle of gemstone coloring

The reasons for the color of gemstones can be basically divided into two categories. One is the color of physical optics, which is a pure physical phenomenon, which is caused by the interference, diffraction and dispersion of precious stones to light; The other is the result of selective absorption of electromagnetic waves caused by electronic transition. The latter can be the color of matter, such as various chromogenic ions, including autologous color and other colors we often say; It can also be structural coloring, such as gemstone coloring caused by various color centers. The selective absorption of electromagnetic waves by gemstones is mainly related to its composition and internal structure. Due to the different types of transitions, the absorbed energy is different, and the final color of gemstones is also different.

Modern material structure theory holds that matter is composed of atoms, atoms are composed of nuclei and electrons, and electrons move outside the nuclei. Quantum mechanics describes the laws of motion of electrons and other microscopic particles. According to the hypothesis put forward by Bohr in 19 13, it is believed that atoms exist in a stable state with certain energy, that is, steady state. Atoms can have many stationary states with different energy values, and each stationary state is arranged according to energy levels to form an energy level. The stationary state with the lowest energy is called the ground state and the rest is called the excited state. Usually atoms or ions are in a stable state, that is, the ground state, when there is no energy radiation; If atoms or ions are affected by external radiant energy, heat energy, electric energy, etc. The electrons outside the nucleus absorb energy and jump to the excited state, but the excited electrons are unstable. After about 10-8 seconds, the electrons return to the ground state, and some energy is radiated in the form of light. In short, gemstones are colored because the electrons in the atoms formed by gemstones have an electronic transition from the ground state to the excited state under the action of energy provided by external irradiation, and selectively absorb light waves with specific wavelengths to show colors. Table 4- 1-3 is a comprehensive summary of the former Soviet scholar пдатоов (1976) and the American scholar Nassau (1983).

Table 4- 1-3 Gemstone Minerals 12 Color Types

It can be seen that the explanation of gemstone color in modern science is mainly based on spectroscopy, and is based on crystal field theory, molecular orbital theory, energy band theory and physical optics theory. The coloring mechanism of these gems will not be introduced here.

Fourth, multicolor.

Polychromaticity occurs in colored anisotropic gemstones, which refers to the phenomenon that the selective absorption and absorption intensity of anisotropic gemstones change with the direction. As introduced in the third chapter, when the light enters the anisotropic gem along the non-optical axis, it will be decomposed into two polarized lights with perpendicular vibration directions. These two kinds of polarized light have different propagation speeds in gems, and both of them interact with gems in the process of passing through them. Because the internal structure of anisotropic gem is different in the vibration directions corresponding to these two polarized beams, the absorption and absorption intensity of light waves are also different in these two directions, which leads to polychromatic. The polychromatic color of a gem can be understood by its optical index. For uniaxial crystal gemstones, dichroism can be observed, and the two main colors correspond to the vibration directions of Ne and No in the optical indicator. For biaxial crystal gemstones, three colors and three main colors can be observed, which are respectively equivalent to the vibration directions of Ng, Nm and Np in the optical index line.

For multicolored gemstones, what our human eyes observe is mixed colors. However, if we use an auxiliary polarizing device, such as a dichroic mirror, we can see the color of the gem in two directions at the same time, and the vibration direction is vertical. When observing the multicolor of gems, some gems may have obvious color and tone changes, while most gems only show different shades of the same color. The expression of multicolor is generally expressed by terms such as strong, obvious or weak according to the hue and depth of color in gemology. See table 4- 1-4 for common multicolor gems.

Table 4- 1-4 gemstone multicolor table

sequential