Gem identification

The identification of gemstones can generally be divided into two major categories: rough and finished.

For the identification of rough stones, it can be divided into field identification and indoor identification. Most of the field identification uses simple tools such as magnifying glasses and knives to preliminarily name gem minerals. Indoor identification mainly uses various means and instruments to further determine the data of gem minerals and provide an important basis for identifying gemstones.

For the identification of finished gemstones, the gemstones determined must be identified without destroying the integrity of the gemstone.

Currently used and easy to master gem identification instruments are as follows:

1. Pen-type spotlight flashlight: used to observe the transparency of thick colored stones. The electric bead of the concentrating flashlight should be concave on the surface of the pen, and it should not protrude from the surface of the pen, otherwise it is not easy to observe.

2. Magnifying glass: It is one of the instruments for magnifying observation of gemstones. The most commonly used is a 10x magnifying glass, and there are several 20 or 30 times. The magnifying glass is a key tool and must-have for gemologists and is easy to carry. It can be used to identify the variety and authenticity of a gemstone. A magnifying glass can be used to observe: (1) surface damage, scratches, and defects of the gemstone. (2) 琢 type quality. (3) The quality of the polishing. (4) Defects and inclusions inside the gemstone. (5) Color distribution and growth lines, etc. At the time of identification, the gemstone should be placed under a strong light of about 2.5 cm from a magnifying glass of 10 times, and the distance should be adjusted slowly until it is clear. It is also important to choose the quality of the magnifying glass. The poor quality will produce pattern distortion when zoomed in.

3. Two-color mirror: Some gems have pleochroism, and the best instrument for observing gems is dichroic mirror. The dichroic mirror is an optical instrument with reasonable structure, low price, small size and simpleness. The dichroic mirror uses a suitable transparent colorless calcite (Ice Island stone) rhombohedron. Due to the high birefringence of the ice stone, the instrument can separate the two plane polarized rays passing through the gemstone. The requirement is that a single crystal gemstone with a transparent color can detect pleochroism, and jade cannot detect pleochroism. The dichroic mirror is mainly used to distinguish ruby ​​and red spinel, red purple dentate; distinguish between blue spinel and fine blue tourmaline; distinguish sapphire and blue synthetic spinel. When using a dichroic mirror to detect a gemstone, you must constantly rotate the gemstone until the two most different colors appear on the window. For the determination of the trichromaticity of gemstones, it must be carefully and repeatedly tested, observed from three different directions, and three colors appear to be trichromatic. Note when testing: Eyes, dichroic mirrors and gemstone samples should not exceed 2-5 mm.

4. Refractor: Refractive index is an important optical constant of transparent gemstones and is the main basis for identifying gemstone varieties. There are two main methods for measuring the refractive index: one is direct measurement, which is measured by a refractometer; the other is relative measurement, using liquid immersion. The refractometer is manufactured based on the principle of total reflection of light. Currently used refractometers are only suitable for gems in the range of 1.36-1.881. The refractive index (N) of gemstone is calculated as the ratio of the propagation velocity of light in air (V1) to the propagation velocity (V2) in a gemstone, that is, N = V1/V2. A homogeneous gemstone in which light travels, with constant propagation speed and equal refractive index, called single refractive index. Heterogeneous gemstones have two readings in the refractometer. The difference between the maximum and minimum refractive index values ​​is called the birefringence rate. The refractometer is one of the most commonly used instruments for gemologists. It is small and easy to use. He can test the refractive index of faceted gemstones, and can also measure the refractive index of gemstones by point measurement.

5. Charles filter: The filter is designed to absorb the specific wavelength of light. It consists of two gemstone accumulators consisting of gelatin filters that pass only deep red and yellow-green light. The filter is small, lightweight and easy to carry. It is especially effective for identifying some dyed gemstones and artificial gemstones. It is very effective for identifying twilight jade. It can identify emeralds and other imitations, but to be accurately determined, it is also considered by other methods. Under the color filter, the emerald appears red or pink, while other natural green gemstones similar to emeralds do not appear red under the color filter.

6. Gem microscope: An important instrument for gemstone magnification observation. It is capable of detecting 10 times the magnifying glass and cannot clearly confirm or observe the external and internal features of the gemstone. The gem microscope can observe the inclusions, cleavage, double crystal lines, growth lines and ribbons inside the gemstone; observe the grinding, polishing and accidental damage of the gemstone; identify the two-layer stone and three-layer stone of the gemstone. The structure of the gem microscope is reasonable, and the auxiliary equipment is fully equipped. The magnification of the gem microscope is large, generally 10 to 70 times. Gem microscopes have two kinds of light sources. Generally, the bottom lamps are used to observe internal defects of gemstones, such as inclusions and cracks. The surface characteristics of gemstones such as fractures, ribbons, and cleavage surfaces are observed by reflector lamps. Gem microscopes are precision instruments that must be used in strict accordance with the operating rules.

7. Thermal conductivity meter: The thermal conductivity meter is designed according to the good heat transfer of diamonds. Most gemstones do not have thermal conductivity or have very low thermal conductivity. Therefore, general thermal conductivity meters are designed to distinguish diamonds from artificial imitation diamond products. They are special instruments for identifying diamonds and other imitation diamond products. The diamond thermal conductivity meter consists of a metal needle probe and a control box. When the tip of the probe touches the surface of the diamond, the temperature is significantly reduced, and the measurement result is displayed by the instrument head signal light or the chirping sound. The thermal conductivity meter is more than ten centimeters long and is easy to carry and extremely convenient to use.

8. Polarizer: A simple optical instrument that is made by the principle that the plane polarized light intersects perpendicularly and the light passes. The polarizer consists of two polarizers, a bracket and a bottom light that are perpendicular to the direction of vibration. It is used to detect the lightness (either homogenous or heterogeneous) and pleochroism of gemstones. In the polarizer that turns on the illumination, turn to observe the change in brightness of the gemstone sample. (1) If the sample is bright and there is no change in brightness and darkness, it may be a cryptocrystalline or microcrystalline aggregate such as chalcedony or jade. (2) If the sample is completely black, there is no change in brightness and darkness, the sample is changed to an angle to continue observation, and if there is still no change in brightness, the sample is homogeneous. Gemstones that are homogeneous are equiaxed and amorphous. (3) If the gemstone is rotated 360°, the gemstone sample undergoes four changes in brightness, which indicates that the sample is heterogeneous. Gems that are heterogeneous are gems in the tetragonal, hexagonal, trigonal, orthorhombic, monoclinic, and triclinic systems. (4) If the sample is rotated under orthogonal polarized light, a dark serpentine, grid-like or irregular phenomenon can be seen, which may be the abnormal interference color exhibited by the homogenous gemstone. At this time, great attention should be paid. . Using a polarizer, it is also possible to detect the pleochroism of a gemstone and to verify the heterogeneity and homogeneity of the gemstone.

In addition, commonly used gem identification instruments include absorption spectrographs, fluorescent lamps, X-ray diffractometers, and electron probes.

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