Plastic is an artificial organic material, which is composed of long-chain polymer molecules. Plastic can be heated or molded to make the imitation gems we need. The earliest plastic used to imitate precious stones was celluloid, a cellulose plastic. Now, celluloid is replaced by a large number of synthetic resin plastics, including phenolic resin, phenolic resin, methyl methacrylate resin, polystyrene resin, polyvinyl chloride resin and so on.

A particularly transparent and bright plastic is polymethyl methacrylate. This material is made of acetone, hydrogen cyanide and methanol. Its methyl acrylate has a carbon-carbon double bond. Under the action of catalyst, one of the double bonds is opened to form long-chain polymer molecules.

Table 4-1-1lists the main types of hard plastics that can be used for gem imitation.

Table 4-1-1/main types of hard plastics

As imitation materials of precious stones, plastics are often used to imitate organic precious stones such as amber, ivory, coral, pearls and shells and some inorganic precious stones (such as opal, turquoise, jadeite and nephrite), and also used to imitate rubies, emeralds, amethysts and diamonds among transparent precious stones.

(B) the nature of imitation gem plastic

1. Appearance characteristics

Compared with glass, plastic is made into imitations by casting process, so plastic imitations often show the unique properties of casting imitations, such as casting marks, concave surfaces, "orange peel" effect, smooth faceted edges and so on.

2. Gloss and fracture

Because the hardness and refractive index of plastics are usually low, most plastics show oily luster to sub-glass luster, while plastics stored for a long time show dull waxy luster. The fracture of plastic imitation is usually shell-like to uneven, showing sub-glass to dull fracture luster.

3. Micro-characteristics

Similar to glass, plastic products often show streamline structure and bubbles of various shapes. When the bubbles reach the surface of the gem, a hemispherical cavity will be formed.

The unique structure of natural organic gem materials imitated by plastics, such as coral, ivory and tortoise shell, is helpful for identification, because the plastic imitation of these gems lacks these structural characteristics.

4. Difficulties

The Mohs hardness of plastic is between 1 ~ 3. Because it is quite soft, it is easy to scratch, wear and form pits.

5. Refractive index

Plastics are usually used to imitate gem materials, and the refractive index is between 1.460 and 1.700. In most cases, the refractive index is in the low value region of this range.

6. Optical characteristics

Transparent plastic imitations often show strong abnormal birefringence effect under cross polarizer, and most of them show serpentine stripes. Stress-induced interference color is also common in cross polarizers.

7. Density

The plastic density of most imitation gems is between 1.05 ~ 1.55g/cm3. But the density is lower than that of saturated saline (1. 13g/cm3), because some fillers will increase the density of plastics (even more than 1.55g/cm3).

8. Thermal conductivity

Plastic imitations have obvious warmth when in contact.

9. Hot needle reaction

When touching plastic imitations with a hot needle, most plastic imitations will melt or burn, usually accompanied by pungent and unpleasant smells, such as vinegar, camphor, formaldehyde, fish, carbolic acid and fruit. These unique smells are helpful to identify organic gems imitated by plastics, such as amber, tortoise shell, coal essence, black coral and so on.

(3) Common plastic imitation gems and their identification.

Plastic has low hardness and poor transparency, and its imitation diamonds, rubies and emeralds can only be used to make cheap fashion jewelry.

Plastic imitations of translucent to opaque gemstones can be used as identification features of plastic imitations, such as jadeite, nephrite, turquoise, lapis lazuli, chalcedony, etc., with obvious mold marks and hemispherical holes left by bubbles on the surface.

Among the plastic imitations that imitate organic gems (such as amber, hawksbill, black coral, coal essence, ivory, shells and pearls), amber imitations are the most realistic, and may even contain seemingly naturally occurring inclusions, such as small insects and other organic substances. It should be noted that amber is condensed from liquid, so it often contains bubbles like plastic. It is very effective to identify plastic imitations of amber by density, that is, amber will float in saturated salt water, while plastic imitations of amber (except polystyrene) will sink in saturated salt water.

Plastic imitation tortoiseshell is mainly used as the material of glasses frames and combs, and plastic imitation pearls are often used as fashion accessories. Over the years, it has been quite popular to imitate spherical seawater cultured pearls and grotesque freshwater cultured pearls with plastics.

The combination of glass and plastic can perfectly imitate pearls. The substrate used is translucent white glass beads (hollow wax-filled glass beads were originally used), and now translucent plastic beads or shell grinding beads are often used instead. These beads are coated with a substance called pearl essence or fish scale essence. The chemical composition of this substance is C5H5ON5, which is extracted from the scales of some fish (such as herring). This substance is mixed into plastic nitrolacquer. After the coating is dried, apply 4 to 9 layers to obtain pearl luster. This kind of pearl first appeared in 1656 and was once called "Paris Pearl", "Roman Pearl" or "Wax Pearl".

At present, other materials, such as mica and copper carbonate crystals, are also used to add to the coating, and sometimes guanine coatings are added to this coating.

Identifying plastic imitations is the same as identifying glass imitations. Pearl imitations made of teeth biting plastic also have a slippery feeling. When magnified, the surface is very smooth, and sometimes cracks can be seen in the deep surface, especially near the borehole. It is also possible to find slightly raised lines on the surface, which are caused by plastic flowing out of the mold.

Shell relief imitations made of plastic are often made of translucent orange plastic as the bottom and white plastic as the grain. Plastics are also used to imitate gems with special optical effects. For example, copper is added to transparent and colorless plastic to imitate the fairly cheap imitation of gold star stone glass. A Japanese-made plastic imitation opal can show real color change, because an improved method similar to Pierre gilson's synthetic opal is adopted, but this method is not made of silicon balls, but made of polystyrene balls arranged closely together. Another plastic with slightly different refractive index can be added between these spheres for reinforcement. Because this material shows real color change, it is difficult to distinguish it from natural opal and synthetic opal by naked eye observation. However, a careful study under the microscope will reveal the extremely characteristic "honeycomb" or "lizard skin" structure, similar to synthetic opal. If there is no inlay, this plastic can distinguish natural opal from synthetic opal by measuring its relative density (about 1. 18). This kind of plastic is covered with acrylic film, and its refractive index is 1.48 or 1.49, which is higher than natural opal and synthetic opal. It can also be found that the hardness of plastic imitations is usually low by testing the hardness with a microhardness tester.