The optical properties of gem minerals include transparency, gloss, color, dispersion, pleochroism, and some special optical effects. These are the effects of gemstone absorption, reflection, transmission, refraction, interference, scattering, and diffraction on visible light. It is closely related to the chemical composition, crystal structure and aggregate structure of gemstones, so it is an important part of gem identification and evaluation.

Gem transparency and luster

Transparency - is the process by which light passes through a gemstone, which is related to the chemical composition and structure of the gemstone. The transparency of a gemstone can generally be divided into three levels:

Transparent: through the gemstone, you can see the objects behind, such as crystals, diamonds, etc.;

Translucent: part of the light can pass through the crystal, but can not see through the back objects, such as high quality jade, moonstone, etc.;

Opaque: Light can't pass, such as malachite.

Gloss - the ability to reflect light on the surface of a gemstone. Its strength is just the opposite of transparency, and the gemstone with high transparency is weak. The luster of a gemstone can be divided into:

Metallic luster: very strong reflection, like a shiny electroplated surface, such as hematite;

Semi-metallic luster: strong reflection, such as magnetite;

Diamond Gloss: The surface reflection is stronger than glass and has a brilliant glare, such as diamonds;

Glass gloss: only reflects a small part of the light, such as glass, crystal, topaz, etc.;

Grease gloss and rosin luster: the surface is like a grease-like reflection (such as a crystal fracture) and a reflection like a rosin surface (such as amber break);

Waxy luster: such as the reflection of serpentine jade;

Pearl luster: reflecting soft and colorful light like pearls;

Silky luster: It is as reflective as silk, and is unique to fibrous aggregate gemstones (such as tiger eye stone).

1. The color, chromaticity and dispersion of gemstones

Visible light is decomposed into red, orange, yellow, green, cyan, blue, and purple bands according to the length of the light wave. The color is the reflection of the difference in the absorption of visible light by different gemstones. If a gem absorbs blue and green light waves in white light, the gemstone has a red hue, which is the result of the mixing of the remaining light waves, also known as subtractive color. To accurately observe the color of a gemstone, be sure to do it under natural light or a standard white light source to avoid false tones caused by the light source. In addition to hue, gemology is also commonly used in terms of saturation and brightness: saturation refers to the degree of color of the color is proportional. For example, emerald and malachite are green, but the former is much brighter than the latter. Brightness is the brightness of color, which is related to the optical properties and processing quality of the gemstone itself. Valuable colored stones often require pure color, saturation and brightness.

Polychromaticity - refers to the characteristic that heterogeneous gemstone crystals exhibit different colors in different directions of crystal due to anisotropy, and have dichroism and trichromaticity. For example, the sapphire crystal is blue-green in the direction of the column and blue in the vertical direction, so it is dichroic; the gems with strong chromaticity can be perceived by the naked eye, but the pleochroism of most gemstones needs special Instruments (such as dichroic mirrors) can be observed.

Dispersion - refers to the decomposition of light produced when natural light is obliquely incident on a medium, such as the use of a prism to break sunlight into seven colors. The honed gemstones cause dispersion, but the degree of occurrence varies depending on the refractive index of the gemstone. The ability of each gem to cause dispersion is called the dispersion of the gem. For example, the diamond has a high dispersion and produces brilliant brilliance, which is very dazzling; the crystal has a low dispersion.

2, special optical effects

The coloring effect is a phenomenon caused by the mutual interference of reflected light waves due to the special arrangement of the internal materials of the gemstone. For example, the pearl has its characteristic rainbow-like soft halo, the so-called "pearl light".

Color-changing effect - refers to a kind of gemstone like Opal, which is similar to the oil painter's drawing board because of the multi-colored phenomenon caused by the internal regular arrangement of layered microspheres to diffract natural light.

Moonlight effect - is a kind of scattering phenomenon of light, such as moonstone (micro plagioclase), due to the internal lattice-shaped twin crystal structure, causing irregular reflection (scattering) of light, forming a white halo with a soft and lovely moonlight. It has a light blue color.

Starlight effect - refers to the phenomenon that the gemstones with curved surfaces form a four-shot, six-shot, and twelve-shot star-like ray under the illumination of light. It is caused by the directional reflection of light from the vertical arc of the inclusions contained in the gemstone, such as star sapphire and star spinel.

Cat's eye effect - refers to a gemstone that is curved into a curved surface. Under the illumination of light, it presents a phenomenon of a silky band like a strip-shaped pupil in the cat's eye. The band varies with the position of the observer. parallel movement. The reason for this is that there are densely arranged equal fibrous inclusions on the vertical orphan plane of the gemstone, which are formed by the reflection spots of the inclusions. Gemstones with cat's eye effect include gold emeralds, tourmaline, apatite, beryl and the like. Among them, the gold emerald is the most obvious and most like a cat's eye, so the "cat's eye" refers specifically to the golden emerald with cat's eye effect.

The color-changing effect refers to the phenomenon that some gemstones can display different colors under different light sources. For example, the green emerald has two red and green light-transmissive areas, which can be under incandescent light with more red light components. Enriching the ruby ​​red, the green color of the gemstone can be enriched in daylight with more green light components, so this gemstone is also called "the stone." Thai green sapphire, etc. can also have a color change effect.

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