To know jewelry, it must be applied to many basics of chemistry, physics, and mineralogy and crystallography. We can only give an overview of the most basic and simple content here.
First, the material composition
We know that rocks are made up of one or more minerals, and minerals are made up of a number of chemical elements. For example, diamond (diamond) is composed of carbon, crystal is composed of silicon dioxide (ie, oxygen, silicon), and opal is composed of silicon dioxide and water (hydrogen, oxygen).
In nature, gem minerals are often not composed of 100% pure substances, that is, minerals often have other elements mixed together, such as amethyst is a crystal containing traces of iron ions, so it is customary to form an essential element of a mineral. For basic elements, such as oxygen and silicon in amethyst, other elements are called trace elements.
In addition, minerals often contain other solid objects such as solids, liquids, and gases that are formed during or after the growth process. They are called "inclusions" and the space occupied by them is "crystal defects".ç‘•ç–µ (Bao Yusan's enamel also includes other cracks, impurities and damage caused by processing).
The presence of trace elements and inclusions often affects the color and quality of gemstones, but it also becomes an imprint of the nature, origin and authenticity of gemstones. It is an important indicator for the research, identification and evaluation of gemstones. For example, emeralds and aquamarins belong to the same kind of minerals called beryl. The former is emerald green because of trace elements containing chromium, while the latter is blue-blue due to iron. The value is naturally far apart. Ruby and sapphire belong to corundum. The basics are all Al2O3, but the ruby ​​contains Cr, the sapphire contains Ti or Fe, making them completely different colors; and the Burmese ruby ​​contains the gas-liquid inclusions of fingerprint-like distribution, the Sri ruby ​​contains zircon and Apatite inclusions, few synthetic ruby ​​inclusions, occasionally see some round, teardrop-shaped bubbles; and in the evaluation of diamonds, the amount of inclusions constitutes one of the important indicators of evaluation, the more inclusions (瑕疵) More, the lower the value of diamonds.
Second, crystals and crystal forms
Crystal - a solid with a lattice structure inside, which has a certain chemical composition, in which the material points (atoms, ions or molecules) are arranged in a lattice structure according to a certain regularity. The well-developed crystal appearance has a certain geometry (including crystal faces, crystal edges), such as stone salt (salt salt), which is composed of chloride ions and sodium ions, and its geometric shape is cubic.
If the space is limited when the crystal grows, it usually does not have a certain crystal form, but the inside of the crystal still has the structure of the crystal, which is called crystal or grain, such as quartz grain in granite. If the composition of a mineral aggregate (such as rock) is visible to the naked eye, it is called a crystalline structure. If the grain is very small and the naked eye is indistinguishable, it is called a cryptocrystalline structure, such as turquoise. The granules are only 1-5 microns in size.
Solid materials that have not undergone crystallization process have no regular distribution of internal particles, and there is no fixed geometry on the outside. We call it amorphous, such as glass and amber.
The crystal has a certain geometric shape. The mineralogists divide the three imaginary lines intersecting at the center of the crystal into crystal axes. According to the crystal axis length, intersection angle and crystal symmetry elements of different crystals, all crystals are divided into seven crystal systems. 47 crystal forms. Different minerals may have the same crystal form, for example, the crystal form of spinel and diamond may be the same; and the same mineral may also have one or several crystal forms, such as diamond having octahedron, tetrahedron and rhombohedron.
Third, the optical properties of gemstones
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.
Fourth, the mechanical properties of gemstones
1, hardness
Hardness - refers to the ability of a gemstone to resist external mechanical action. Hardness is divided into relative hardness and absolute hardness. The relative hardness is calculated using the Mohs hardness program, which consists of ten common minerals. Grade 10 hardness from small to large is: 1 talc; 2 gypsum; 3 calcite; 4 fluorite; 5 apatite; 6 feldspar; 7 quartz; 8 topaz; 9 corundum; The test method is to use gems to scribe these minerals. When the marks are drawn, the gems are harder than the minerals. For example, the jade can be obtained with gold emeralds, while the relative hardness of the corundum is about 8.5. In addition to standard hardness minerals, other simple tools are often used to test hardness, such as nail hardness of about 2.0 to 2.5, copper key of 3.0, small steel knife of 5.0 to 5.5, and glass of 6.0. It is important to note that you should not use the jewels that have been honed to scratch the gems. Absolute hardness is measured using a microhardness tester, but is not commonly used in practice.
Except for equiaxed crystals and amorphous gemstones, most of the gemstones have different hardness in different crystal directions due to their anisotropic properties. The most obvious is kyanite, and the crystal face hardness on the parallel cylinder surface is 4.5, the hardness in the direction of the vertical cylinder is 6.5. Using hardness to identify gemstones is a very effective method.
2, cleavage, fracture, cracking
Cleavage - refers to the nature of gem crystals cracking along a certain crystal direction when struck by external force, and the flat surface of the crack is called the cleavage plane. Mineralogy divides the degree of difficulty in cleavage into five levels, namely, extremely complete, complete, medium, incomplete, and extremely incomplete. For example, the muscovite has a completely complete set of cleavage, and the muscovite can be stripped into a very thin sheet along its direction; the rosewood has complete cleavage, while the garnet, quartz and tourmaline are not cleavable. Some gems can be identified by cleavage, but cleavage is also a disadvantage that limits the quality of the gemstone and its processing.
Fracture - refers to the section of the gemstone that is ruptured open after being hit, often uneven or uneven. Gem crystals that are difficult to produce cleavage are prone to breakage. If quartz (crystal) is hit, it will easily produce a shell-like fracture.
Cracking - refers to the fact that some gemstones with twin crystals are cracked along the twin-crystal joint after being struck by force. For example, the inside of the moonstone often cracks along the twin plane of the lattice twin.
3. Toughness and brittleness
The ability of a gemstone to resist external damage is called the toughness of a gemstone, and a gemstone with a small toughness is brittle. Toughness (sex) does not necessarily have a fixed ratio to hardness.
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