(3) Filling and Identification of Nephrite

Artificial methods, such as organic glue, resin, and plastic, fill loose or cracked Hetian jade. The nephrite after filling treatment has the following characteristics:

① when observed with a magnifying glass or microscope, the filled jade shows a difference in surface gloss between the filled parts and the main jade; sometimes bubbles can be observed at the filling sites.

② Infrared spectroscopy testing often reveals characteristic peaks of the filling material; using luminescence image analysis (such as ultraviolet fluorescence observation), the distribution state of the filling material can be observed.

③ If the filling material is wax, using a heated needle to probe the surface of the nephrite may result in wax being exuded from the surface.

(4) Assembly and Identification of Nephrite

The assembly of nephrite is mainly used for surface or decorative carving parts. The main body of assembled nephrite is usually made of white jade material with a oily luster and weak glassy luster. Nephrite can be carved and generally has a brownish skin.

The surface after assembly is semi-transparent, with relatively weak luster. However, due to its small volume, it is not easily noticed by people, resembling high-quality nephrite pebble with a sugar color. When paired with exquisite carving techniques, it has an aesthetically pleasing shape.

Upon careful observation of the exquisite carving parts, the color boundary at the junction of the surface and the main body is obvious, with the surface color distributed along the boundary between the main body and the surface (Figure 6-15).

The common methods for agate include heat treatment and dyeing. Heat treatment, also known as color modification, is commonly referred to as “burning red” and is the most frequently used optimization treatment method for agate. Agate with heat treatment has bright colors and good stability, classified as optimized and named directly as agate.

(1) Agate Heat Treatment

① Principle: The red color of agate is mainly due to trace components Fe³⁺ that cause coloration. At high temperatures, the coloring ions Fe²⁺ are oxidized to Fe³⁺, increasing the  ratio of Fe³⁺ and making the red color of agate more vivid.

② Equipment: The most important equipment for agate heat treatment is heating equipment; commonly used heating devices are coal furnaces and electric furnaces. Choose the appropriate heating equipment based on the agate material; the pros and cons of coal furnaces and electric furnaces are as follows:

• Coal Furnace: It is not easy to control the temperature, which may lead to cracking, melting, and insufficient flame, but it has a good insulation effect.
• Electric Furnace: It is easier to operate, and the temperature can be manually controlled for heating and cooling; the time at the highest temperature can also be controlled, but it is generally not convenient for batch production and has a smaller capacity.

③ The heat treatment temperature of agate is relatively high, generally requiring 1300 -1600℃. Heating should be done slowly to prevent fissures caused by excessive heating speed.

When heat treating agate, the “timing” should be based on the agate’s original color, and the heat treatment’s maximum temperature must be controlled accurately. The process is not complicated; as long as the “timing” (optimal heat treatment temperature) is mastered, agate with varying degrees of red tones can be fired into bright red colors of different depths.

Heat treatment of agate belongs to optimization and does not require identification. Heat-treated agate is directly named using the name of natural gemstones. Compared to natural agate, heat-treated agate has more vivid colors and higher saturation, but the overall texture of the agate is dry, with poorer moisture content.

(2) Agate Dyeing

The dyeing of agate involves immersing dye materials into the pores of the agate, resulting in overall coloration. The dye does not react with the components of the agate SiO₂ but is merely a mechanical deposition. There are several requirements during the dyeing of agate:

① Raw Materials:

Before dyeing agate, selecting raw materials that are easy to dye is necessary. The agate used for dyeing must meet the following requirements:

• Structure: The structure of the agate raw materials used for dyeing should have a low density and micro-pores. Dyes are not easily absorbed into the fissures of high-density agate, making it difficult to achieve vibrant colors. an electron microscope study on agate structure and proposed the principle of “three dyeing, five not dyeing” for agate dyeing.

“Three-color” refers to the fact that agate has the following three easily dyed structures: herringbone-shaped fibrous structure, wavy fibrous structure, and multi-generational slender fibrous structure.

“Five-colorless” refers to the fact that agate has the following five structures that are not easily dyed: non-directional short fibrous granular structure; flower-like speckled granular structure; quartz’s allotriomorphic uneven granular structure; central and core quartz particles; coarse crystallization, clear boundaries at the edge of the grains, close intergranularity, and no microporosity cannot form a channel grain.

• Color: The raw material requirement is for light-colored or white varieties that must be cleaned thoroughly. The color of the agate raw material to be dyed black should be a bit darker.
• Thermal history: The agate to be dyed must have been removed, as roasted agate is difficult to color.

② Equipment:

The equipment required for agate dyeing is simple, as it immerses the dye. A glass container for soaking, a thermometer, a drying oven, and a muffle furnace are needed.

③ Dye

• Easily soluble in water or other reagents.
• Can react with some chemical reagents (fixatives) to form insoluble precipitates in water and alcohol, and the residues are colored.
• The generated colored substances should have good stability and not be decomposed or destroyed by sunlight, air, water, oxidants, or reductants.

④ Methods of dyeing and common dyeing agents

• Traditional methods: Previously, organic dyes were commonly used. In recent years, inorganic pigments have gradually replaced organic dyes due to their bright colors and stable physical properties.

For black agate, the sugar-acid process is still used to dye the agate black, known as “Black Anils. ” The sugar-acid process involves soaking sugar into the pores of the agate and then heating it with concentrated sulfuric acid to carbonize the sugar and form a black color.

• Some current methods abroad: Red: Soak the agate in a Fe (NO₃)₃ solution for about four weeks, allow it to dry slowly, then heat to decompose, producing Fe³⁺ that turns the agate red.

Prussian blue: Soak the agate in potassium ferrocyanide K₄[Fe (CN)₆] solution for about two weeks, then place it in iron sulfate  [Fe₂(SO)₄)₃] solution, soaking for about five days, where Fe³⁺ reacts with potassium ferrocyanide to generate Prussian blue precipitate in the agate fissures. The reaction formula is as follows:

4Fe³⁺ + 3[Fe(CN)]₆^4- 一 Fe₄[Fe (CN)₆]₃ ↓     （6-1）

This reaction is very sensitive, and the generated blue is very bright.

Tururnbull’s Blue: Soak white agate in potassium ferricyanide K₃[Fe (CN)₆] solution for about two weeks, then take it out to dry and place it in FeSO₄ solution for 3-5 days. The Tururnbull’s Blue residue generated from the reaction between Fe²⁺ and potassium ferricyanide settles in the fissures of the agate, but the color is darker.

3Fe²⁺ + 2[Fe(CN)]₆^3- 一 Fe₃[Fe (CN)₆]₂ ↓     （6-2）

Prussian Blue and Tururnbull’s Blue have similar colors, but Prussian Blue is slightly lighter than Tururnbull’s Blue.

Blue-green: Soak agate in chromate (Na₂CrO₄, K₂CrO₄)  or dichromate (K₂Cr₂O₇ or Na₂Cr₂O₇) for1-2weeks, then take it out and place it in a container containing (NH₄)₂CO₃, gently heat it, maintain for about two weeks, and then heat again, the agate turns blue-green. The reaction formula is as follows:

K₂Cr₂O₇ + (NH₄)₂CO₃ →(NH₄)₂Cr₂O₇ + K₂CO₃  (6-3)                                       

 (NH₄)₂Cr₂O₇ →Cr₂O₃ + N₂ ↓ +4H₂O              (6-4)

Black: Soak agate in silver nitrate solution for 1-2 weeks, then place it in (NH₄)₂S solution to soak; the resulting black precipitate Ag₂S makes the agate appear black. The reaction formula is as follows:

2AgNO₃ + (NH₄)₂S →Ag₂S ↓+2NH₄NO₃           (6-5)

• The methods used domestically: the methods used domestically for agate dyeing technology are relatively mature, allowing agate to be dyed in different colors. In addition to the commonly seen red, green, and purple, agate can be dyed in other colors, such as brown, cherry red, peach red, and apple green. The operation method is similar to the abovementioned methods, but the chemical reagents used differ. Dyeing is a major optimization treatment for agate, and it can enhance or change the color of agate.

According to the principles of agate dyeing, there are three types of dyeing methods:

The coloring agent is immersed in agate, followed by heating, decomposition, or redox reactions to generate colored oxides. For example, to dye agate apple green, a nickel nitrate solution can be used to soak the agate, followed by heating to allow nickel ions to penetrate the fissures in the agate.

Two chemical reagents that can react chemically to generate coloring agents are sequentially immersed in agate in two stages. The generated coloring agents are subjected to heat treatment, which can decompose into colored oxides. For instance, in the blue-green dyeing method, potassium dichromate reacts with ammonium carbonate to generate ammonium dichromate, which decomposes upon heating to produce chromium trioxide as the coloring agent.

Two chemicals that can react chemically to produce a dye are applied to agate in two separate treatments. The dye formed by the reaction is then subjected to heat treatment, which can break it down into a pigmenting oxide. For example, the blue-green dyeing method involves the reaction of potassium dichromate with ammonium carbonate to produce ammonium dichromate, which is then heat-treated to produce chromium(III) oxide, the pigment.

First, immerse a dye into the interior of the agate and then soak it in a fixing agent, allowing the dye to react with the fixing agent to produce a poorly soluble colored compound, thereby coloring the agate.

This method does not require high-temperature heating, and the generated precipitate has good stability.

⑤ Identification of dyed agate

• Finding differences in color: Different color tones: organic dyes are bright and prone to fading. In contrast, inorganic pigments’ colors are closer to natural products, but careful observation can also reveal differences. Below are distinctions based on three common colors:

Natural red agate is a pure red color. In contrast, artificially dyed red agate has iron ion compounds added, resulting in a red color with a yellowish tint.

Natural blue agate is produced in very small quantities, mostly in gemstone blue, and often exhibits varying degrees of banding. Artificially colored blue agate appears violet (cobalt blue) due to the addition of cobalt salts, which gives it a bluish-purple hue, with very few cases showing gemstone blue coloring.

The colored green agate is very close in color to the natural variety. However, upon closer inspection, the natural variety is a soft onion green, while the colored green agate is a bright emerald green with higher saturation.

• Finding differences in structure: Since dyed agate is colored by soaking and drying it with pigments, the pigments settle in the pores of the agate, and under magnification, uneven color spots can be found in the fissures and pores.

Generally, a tenfold magnifying glass is sufficient for identification, while fine-dyed products need to be observed under a gem microscope. Dyed and heat-treated agate can be observed to have “nail marks” on the surface.

Natural red agate does not have “nail marks, ” and the coloring particles in the agate are red, dot-like iron inclusions, with diffusion phenomena not being obvious or absent. The dyed and heat-treated red agate surface can show “nail marks, ” concentrated in specific areas with varying degrees of color, structure, and transparency, showing uniform color distribution and blurred band boundaries (Figure 6-18).

(5) Methods and Identification Features of Synthetic Opal

Currently, most synthetic opals are synthesized using the Gilson synthesis method. The main synthesis process is as follows:

① Formation of Silica Spheres: Add a medium-strength alkali (such as ammonia) to the organic silicon compounds that diffuse into small droplets in a mixed solution of alcohol and water, turning the organic silicon compounds into silica spheres. The purity, concentration of the reagents and stirring speed must be carefully controlled to generate spheres of the same size and to obtain different types of opal varieties as required, with a diameter between 200 and 300nm.

② Precipitation: They continuously precipitate After forming silica spheres. Once precipitated, these spheres automatically arrange themselves tightly. This stage is relatively slow and may take more than a year.

③ Compaction and bonding: This process is the most difficult and key to producing high-quality opal materials. Silica spheres are covered with liquid, and equal hydrostatic pressure is applied to the spheres in all directions to avoid structural changes; finally, the silica spheres may be bonded with added colloidal material, or the materials are sintered at a certain temperature.

Finally, the formed opal is cut and polished to display a better play of color effect.

Identification of synthetic opal versus natural opal:

① Structure:

The color spots of natural opal are two-dimensional, with a silky appearance, elongated in one direction; they are irregular thin sheets; the color spots have a gradient relationship with blurred boundaries; the color spots have fibrous or striped structures in one direction (Figure 6-25).