First, the cat's eye effect.
(1) definition
Under the irradiation of parallel light, a bright light band will appear on the surface of some cut and polished jewels and jade, and this light band will move with the rotation of the sample or light, which is the so-called cat's eye effect.
(B) the mechanism of cat's eye effect
The conditions of cat's eye effect are as follows: ① There must be a group of dense, parallel and oriented fibrous, needle-like or flaky inclusions or some special structures (such as solid solution exsolution structure) in the gem; (2) The bottom surface of the globoidal gem should be parallel to the plane of the inclusion; ③ The height of the curved gem is consistent with the height of the focal plane of the reflected light. Pay attention to making the bright line parallel to the long axis of the gem.
Cat's eye effect is caused by the refraction and reflection of visible light by a group of dense, parallel and directional inclusions or directional structures in gems. As shown in figure 1-3-24, the S plane is NM containing inclusions, perpendicular to the longitudinal section of the arc-shaped gem of the "eyeliner" of the cat's eye. When the light from a light source shines on a gem, the following happens.
Figure 1-3-24 schematic diagram of cat's eye effect
1) The light incident along point O, that is, the light incident along the normal of NM, enters the gem directly without refraction.
2) The light incident along point A and point B is decomposed, and part of the light is refracted into the gem, which is refracted according to the law that the light deviates from the normal direction when it enters the dense medium. The refracted light entering the gem from point A and point B is inclined to its normal direction, that is, it is refracted to the normal OO' direction of NM.
3) The light entering the gem from point A and point B will be decomposed again when it reaches point Ga and point Gb on the inclusion NM, some light will be refracted out of the NM surface, and the other light will be reflected and reach the surface of the arc gem again.
4) Influenced by the curvature of the cambered surface, the incident angle of incident light from point Ga to point Gb gradually increases. According to the law of reflection, the reflection angle is equal to the incident angle, and the incident light, reflected line and normal line are all in the same plane. Therefore, the light reflection angles γ Gb > γ Ga of two points GB and GA are in the S plane, and the two reflected lights intersect at one point in the S plane.
5) By analogy, light rays entering two points a' and b' on the other side of the arc surface are also reflected and intersect at one point. When the height of the globoidal gem is appropriate, these four reflected rays can intersect at a point on the globoidal surface. When there are many inclusions arranged in parallel in the gem, the trajectory of reflected light generated by inclusions at the intersection of gem surfaces forms the eyeliner of cat's eye.
(3) The relationship between the height of globoidal gem and the width of eyeliner.
The width and brightness of the "eyeliner" of a gem with cat's eye effect are influenced by the refractive index of the gem itself and the height of the arc gem. For a specific gem, its refractive index value is fixed, and the focal plane height of the reflected light reflected by the inclusion is also fixed (see figure 1-3-25).
Only when the height of the arc-shaped gem is consistent with the height of the focal plane of the reflected light can the "eyeliner" of the gem present a narrow and bright light band. When the height of the arc-shaped gem is lower than the focal plane of the reflected light, the "eyeliner" of the gem appears as a wide and sparse band, and the brightness of the band decreases. Generally speaking, the higher the refractive index of a gem, the lower the focal plane of the reflected light of the inclusion. Therefore, the higher the refractive index, the lower the cambered surface height, the lower the refractive index and the higher the cambered surface height of a gem with cat-eye effect, so as to make the cat-eye effect more obvious.
(D) "Eyeliner" swing reason
For gemstones with cat's eye effect, with the swing of the gemstone or the swing of the light source, the "eyeliner" also swings correspondingly and moves against the direction of the light source. This is because when the light source moves from the top of the gem to the side, the incident angle of the light increases relative to the normal, and the angle of the reflected light also increases, resulting in the corresponding shift of the focal plane of the reflected light.
(5) Incorrect handling causes incorrect eyeliner.
When the bottom surface of the arc-shaped gem is inconsistent with the plane of the inclusion and inclined, the focal plane of the reflected light of the inclusion moves to one side of the arc-shaped gem. Therefore, when the globoidal gem is placed horizontally, the position of the "eyeliner" is not in the center (Figure 1-3-26).
Figure 1-3-25 Relationship between the height of globoidal gemstones and the width of eyeliner.
(a) When the height of the arc gem coincides with the focus of the internal reflection line, the "eyeliner" is narrow and bright;
(b) When the height of the arc gem is lower than the focal plane of the internal reflection line, the "eyeliner" becomes wider and thinner.
Figure 1-3-26 Causes of incorrect eyeliner due to improper handling.
Nm-inclusion surface;
O'-the exposed position of eyeliner; O-the highest point of a spherical gem.
Second, the starlight effect
(1) definition
Under the irradiation of parallel light, two or more intersecting bright lines appear on the surface of some cut and polished jewels and jade, which is called starlight effect. Each bright band is called a star line, and there are usually two, three or six more stars. It can be called four-shot (or cross), six-shot star line or twelve-shot starlight respectively. Starlight effect is mostly caused by two, three or six groups of inclusions arranged in dense parallel orientation.
Conditions and formation mechanism of starlight.
Gems that can produce starlight must contain two or more groups of directional inclusions or directional internal structures, and the bottom surface of the arc gem is parallel to the plane where these inclusions or structures are located. The formation mechanism of starlight effect, like cat's eye effect, is caused by the refraction and reflection of visible light by directional inclusions or structures in gems and gems. The difference is that in the starlight effect, inclusions or structures are not limited to one direction, and these inclusions are distributed at a certain angle. The starlight effect is the result of several groups of inclusions and light. Ruby mineral is called corundum, and its crystal form is hexagonal column. On the plane perpendicular to the Z axis of the crystal axis, there are often three groups of fine needle-like rutile inclusions, which intersect at an angle of 60.
If the star line in the starlight gem is decomposed, the situation will be much simpler. As can be seen from Figure 1-3-27, the formation mechanism of each star line is the same as that of the eyeliner in the cat's eye effect, that is, the inclusions in all directions in the figure can form bright light bands on the surface of the arc-shaped gem through refraction and reflection, and the extension direction of the light bands is perpendicular to the arrangement direction of the inclusions that make up it, that is, the R 1 light band is formed by r 1 In ruby, the rutile in r 1, r2 and r3 cross each other at an angle of 60, so the arrangement of rutile in r 1, r2 and r3 on the longitudinal axis plane can be abstracted as three sides of an equilateral triangle, and the three bright bands formed by them can be abstracted as perpendicular bisector of each side of the equilateral triangle. According to the theorem of the perpendicular line of the triangle, these three bright bands must intersect at one point. In a perfect cut starlight gem, this intersection point occupies the highest point of a globoidal gem. After the three bright bands intersect, the six star lines emitted by the intersection point will rotate in the opposite direction with the rotation of light.
The reason for the twelve stars is the existence of the above two groups of starlight combinations. For example, the sapphire with double starlight in Shandong, China, consists of two groups of six starlight, and the two groups of starlight intersect at an angle of 30 to form twelve starlight. When the colors of the two groups of starlight are the same, the intensity of the light band can be the same, or it can change alternately. These two groups of starlight sometimes have different colors, and the common combinations are yellow-green, yellow-brown, yellow-blue, blue-green and so on.
Figure 1-3-27 the cause of ruby starlight
Ruby crystal; (b) r 1, r2 and R3 arranged in rutile in a plane perpendicular to the z axis; (c) The relationship between the star line (R 1, R2, R3) and the inclusion: r1⊥ r1,R2 ⊥ R2, R3 ⊥ R3.
Third, change the color.
(1) definition
The special structure of gem produces color through the interference and diffraction of light, and the color changes with the change of light source or observation angle. This phenomenon is called discoloration.
(B) the principle of color change
The color change is caused by the interference and diffraction of light. According to Young's double-slit experiment (Figure 1-3-24), when the light emitted by the light source is incident along the arrow direction, a point light source is formed at the slit to ensure that the properties of the light reaching the S 1 and S2 slits are completely the same. When light with the same properties passes through two slits (namely Young's double slit) of S 1 and S2, if the light source is monochromatic, the optical path difference between the two sub-wave sources in a certain direction is even multiple of half wavelength, which will be enhanced when they meet in space, and bright stripes EE' will be displayed on the screen; When the optical path difference of two sub-wave sources in a certain direction is odd times of half wavelength, they cancel each other when they meet in space and show dark stripes on the screen EE'. If the light source is white composite light, the result of diffraction and interference is that the stripes of monochromatic light in white light will be arranged in sequence according to the wavelength, with white light in the middle, purple near the white light, blue and green in sequence, and red at the farthest, which are symmetrically distributed up and down.
Figure 1-3-28 Schematic diagram of double-slit interference experiment
Young's test is the interference between points of a one-dimensional grating, but it is often complicated to encounter plane interference in two-dimensional space and interference in three-dimensional space in gems.
(3) Color characteristics of opal
Opal is a typical gem with color-changing effect, and its color and stain change characteristics can be summarized as follows.
1) Some gray opals do not show color spots, but only show blue albumin stone light.
2) Some opals only show blue and green spots on a gray-white substrate, that is, only short-wavelength spots.
3) Some advanced opals display all the color points in the visible spectrum from purple to red on a white or black substrate.
4) The arrangement characteristics of the color spots in the same opal are mixed, and the colors of adjacent color spots are not arranged according to the color sequence of the visible spectrum. That is, the red stains can be directly adjacent to the green stains, or several red stains with almost the same color can be scattered in the gray-blue matrix.
5) Color characteristics of the stain The colors in the same stain are not very uniform, and adjacent colors arranged in spectral color order can appear at the edge of the stain. For example, orange, yellow and green bands can appear in turn at the edge of the red dot. When the opal or light source rotates, the color of the same spot will change according to the spectral color sequence. For example, when the red dot rotates, with the increase of rotation angle, the red dot can turn orange and orange yellow in turn.
(D) The relationship between the color change and the structure of opal.
The chemical composition of opal is SiO2 nH2O. In the structure of opal, silica is almost equal in size and regularly arranged in three-dimensional space. Generally, there are six octahedral cavities and eight tetrahedral cavities around any silica sphere. The octahedral gap is between 0.4 14 and 1 times the diameter of the ball. The size of the tetrahedral gap is between 0.225 and 0.5 times the diameter of the sphere. In this way, the opal structure forms the most typical natural stereoscopic grating. Here, the gap between the silica sphere and the sphere is equivalent to diffraction unit and grating constant, respectively.
The special structure of opal determines its color-changing ability and characteristics. The diameter of the ball, the distance between the balls and the observation angle directly determine the color of the spots in the opal (as shown in figure 1-3-29).
Fig. 1-3-29 opal discoloration effect
According to Bragg formula (N 1λ = 2nd sinθ), when light is incident vertically, θ is 90, that is, sinθ = 1, and when n 1= 1, λ = 2n2d, N2 =1.40. Because white visible light λ is in the range of 700 ~ 400 nm, the gap diameter that can generate diffraction conditions should be from d=700÷2.9=24 1nm to d=400÷2.9= 138nm. If: (1) when the gap distance of the sphere is between 138 ~ 24 1 nm, monochromatic light of all wavelengths in white light can be allowed to pass through to form a color opal; (2) When the gap distance of the sphere is between 138 ~ 204 nm, only five spectral colors, purple to yellow, are allowed to pass through, forming a multicolor opal; (3) When the gap distance of the sphere is between 138 ~ 176 nm, only three spectral colors of purple, blue and green are allowed to pass through, forming a three-color egg white stone; ④ When the clearance distance of the sphere is between 138 ~ 165 nm, only violet and blue light are allowed to pass through, forming a bicolor or monochromatic opal.
The research shows that the accumulation of silica spheres in natural opal is not completely uniform. Uniformly stacked spheres that generate diffraction only exist in a small area from 1mm to less than 1cm. Each uniform small area constitutes an independent three-dimensional diffraction grating. The ability and diffraction of the small grating to allow visible light to pass through determine the size and color characteristics of the corresponding color points of the grating. This explains why the colors of adjacent spots in opal do not follow the color sequence of visible spectrum. The reason why adjacent color rings appear at the edge of each stain is the same as the reason why the color of the stain changes in the visible spectral color sequence when the opal rotates. The fundamental reason is that the observation angle has changed. With the rotation of opal, that is, the change of observation angle, people see another color in the diffraction pattern of small grating, that is, the color corresponding to another diffraction angle θ.
Fourth, the discoloration effect.
The phenomenon that the color of gem minerals changes with the spectral energy distribution of incident light or the wavelength of incident light is called discoloration effect. Not all gems can produce color-changing effect. Only when the visible light absorption spectrum of the gemstone meets certain conditions can the gemstone have a color-changing effect. Take metamorphic rocks as an example (see Chapter 4 for discoloration mechanism). The chemical formula of metamorphic rock is BEAl2O3, and the chromogenic impurity ion is Cr3+. The energy absorbed by the outer D electron transition of Cr3+ ion is 2. 17 e V, which is between ruby (2.25e V) and emerald (2.04e V). In the visible region, red light and blue-green light are transmitted in metamorphic rocks. If the stone is illuminated by fluorescent lamp, it will appear blue-green; Stones will appear red under the irradiation of incandescent lamps (Figure 1-3-30). Besides metamorphic rocks, sapphires and garnets can also have color-changing effects.
Figure 1-3-30 principle of discoloration effect