At present, there are many varieties of opal that can be seen in the jewelry market, including natural opal, dyed opal, combined opal, injection opal, oil-injected opal and synthetic opal. The identification methods of several common varieties are introduced as follows:
Due to the fascination of the opal gemstones and the limited production area, many imitations have entered the market. However, the formation conditions of opal stone are difficult to imitate, so the general imitation is not ideal and easy to identify. Now choose its common imitation opal narrative as follows.
(1) stained opal
Dyeing opal, use the sugar liquid to blacken the inferior white opal, and make the black opal with strong color. Its opal stone material is generally white in color, lighter in color and more porous. When dyeing, first heat the opal stone, then soak it in sugar liquid or orange juice with high sugar content. When it is dry, soak it in concentrated sulfuric acid solution to make the sugar carbonized and blackened. The sugar liquid provides a black background for the color change of the white opal, and the color film appears bright and lovely.
The black color of the opal is often deposited in the gaps of the color chips or pellets. The magnifying glass and the top light source observe the gems with black dust spots, which appear to be contained in the mosaic pattern. In appearance, the color of the gemstone has a raindrop-like depth. Due to the limited penetration depth of the dye, if the dyed black opal is cut, the original appearance of the white opal can be revealed. Since the blackening of the dyed black is removed by the cutting, the dyed opal is firstly cut and then dyed, and in the identification, the trace of the polished surface can be found and identified.
(2) Injection opal
Injection molding opal is to inject plastic into the natural low-grade opal, so that it is black or white, in order to achieve bright color and outstanding color.
Injection opal is the first to increase transparency compared to natural opal, and secondly to reduce the density of gemstones. The natural opal stone has a density of 2.22 g/cm3 and has a density of less than 1.99 g/cm3 after injection molding. After injecting black plastic, the opal is black in color, and it looks like a black bundle of flowing growth under the microscope, similar to the inclusion in the synthetic emerald.
(3) Oil injection opal
Oil-filled opal is the injection of oil or wax into the natural low-grade opal to add luster to cover up the crack. From the appearance, the surface of the oil-filled opal is smooth and has a waxy luster.
The main difference between oil-filled opal and natural opal is the wax or oil on the surface. The test method is simple: heat the needle and place it on the oil-coated opal. When the oil or wax is hot, a ball-shaped oil droplet or wax droplet is formed on the surface of the stone.
(4) Synthetic Opal
Synthetic opal stone, according to the composition structure of natural opal, synthetic artificial opal spherules, and can show the color, luster and color of natural opal stone.
There are three differences between synthetic opal and natural opal:
A. The natural opal color film is spindle-shaped, the middle two sides are thinner, and the synthetic opal color is triangular, expanding in three directions.
B. Synthetic Opal's color film has a hexagonal honeycomb structure, and the color piece constitutes a mosaic pattern, and has a clear mesh structure.
C. Synthetic opal has large pores and strong water absorption.
(5) Glass imitation opal
Glass imitation opal is the addition of colored flakes to the molten glass to create a replica similar to natural opal.
The weakness of glass imitation opal is mainly characterized by the appearance of glass products: glass luster, shell-like fracture. Its refractive index (1.49-1.52g/cm3) and density (2.4-2.5g/cm3) are higher than natural opal (refractive index #1.45, density 2.15g/cm3). The glass imitation opal has no gaps, no water absorption, and the tongue has a slippery feel and does not stick to the tongue. Because it is an artificial color film, the glass opal is magnified and observed under the illumination of a vertical strong light source, and a hexagonal honeycomb structure can be seen.
(6) Plastic imitation opal
Plastic imitation opal, with two-layer structure. The interior is made of polystyrene, and the exterior is covered with a layer of acrylic resin, which is made into a mosaic pattern of needle-like flames, which is very similar to the natural fire opal.
The density of the plastic imitation opal is 1.21-1.26 g/cm3, which is much lower than the natural opal (density 2.15-2.23 g/cm3). I feel light in my hand. Placed in glycerin (density 1.26g/cm3), the plastic imitation opal floats, while the natural opal sinks. The plastic imitation opal hardness is 1.5-3, while the natural opal hardness is 5-6.5, which is very different. The ordinary sewing needle can be used to draw traces on the plastic imitation opal, but not on the natural opal. The plastic imitation opal is semi-transparent light blue opalescence under the illumination of incandescent light, and it is pink-orange opal under the irradiation of transmitted light. There is no light and dark change under the polarizer, and there is abnormal interference color.
(7) Combination Opal
The combination opal is a two-layered stone of Opal and three layers of opal. The key to identification is to look at the waist or the side of the gemstone. The different layers are different in color, gloss, color, transparency, etc., that is, there are signs of stitching, or there are bonding wires, glues or bubbles.
A. Opal two-layer stone: the top surface is of good quality opal, the bottom is lined with black agate, black chalcedony, inferior opal or opal surrounding rock, bonded with black glue.
B. Opal three-layer stone: the top surface is made of colorless quartzite or glass and is ground into a curved surface. The middle part is also curved with natural opal, bonded with the top surface with colorless glue, and then used underneath. Colored gummy on black chalcedony, glass or inferior opal.
The identification of the combination opal is mainly to find the joint surface. It is relatively simple to detect the combination of gemstones without inlays. Use the naked eye or magnifying glass to observe the change of color, gloss, transparency and the presence or absence of bubbles on both sides of the joint. In addition, the glue is relatively soft and can be judged by fine needle exploration. However, it is more difficult for the combination jewel of the bottom of the package. It can be inspected from the countertop with a microscope or strong light. If you can't find the difference between the two sides of the joint, you can use fluorescent lamps to detect the difference between the fluorescence displayed by the gemstone and the viscose.
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