What Is Turquoise?

What Is Turquoise?

Turquoise is a type of jade material and is one of the ancient gemstones in China. Turquoise has a fine and soft texture, moderate hardness, and a delicate color. However, there are significant differences in color, hardness, and quality. It can generally be divided into four major categories: porcelain turquoise, green turquoise, frothy turquoise, and iron-wire turquoise.

 

Turquoise: Turquoise, also known as "Turkish Jade," has a long history of development and utilization. It is believed that ancient Egyptians mined turquoise in the Sinai Peninsula. A pair of golden turquoise bracelets found on the arms of Queen Zer's mummy from ancient Egypt, dating back 6,000 years, is considered one of the most precious turquoise artworks in the world. In foreign cultures, turquoise is often regarded as a symbol of happiness and good luck, believed to ward off evil and disasters. It is said that those who see turquoise as the first thing in the morning will have a lucky day.

 

In ancient Chinese history, evidence suggests that even in the primitive matriarchal society, there were turquoise ornaments. Turquoise artifacts have been found in vessels from a 5,000-year-old primitive society burial site in Sunjiazhai, Datong, Qinghai. In the late 13th century, Italian explorer Marco Polo visited Tibet and witnessed Tibetan women wearing turquoise jewelry. The term "绿松石" (lǜ sōng shí, turquoise) is first documented in the Qing dynasty literature "清绘典考图" (Qīng Huì Diǎn Kǎo Tú), which mentions "Emperor's morning pearls and miscellaneous decorations: Coral for the Sun Altar, turquoise for the Moon Altar." In his work "石雅" (Shí Yǎ), Zhang Hongzhao explains, "It might resemble a pine cone in shape, and its color is close to pine green, hence the name." It is said that ancient Persia was a major source of turquoise, often imported to Europe via Turkey, which is why it's known as "Turkish Jade." In Chinese texts from the Yuan dynasty, turquoise was referred to as "甸子" (diàn zǐ). According to Zhang Hongzhao, turquoise had various names before the Yuan dynasty, such as "碧殿" (bì diàn), "碧甸" (bì diàn), "琅矸" (láng gān), and "瑟瑟" (sè sè).

 

Chemical Composition:

The chemical formula of turquoise is CuAl6(PO4)4(OH)8·5H2O. Turquoise is a hydrous phosphate of copper and aluminum; iron can substitute for copper, resulting in a light green color. Turquoise may also contain quartz, kaolinite, apatite, limonite, and solid black components from asphalt.

 

Physical Characteristics:

Turquoise exhibits a waxy to glassy luster and is translucent to opaque. Its color ranges from light to medium blue, bluish-green, teal, and green. Persian turquoise is bright, semi-transparent, and uniform, displaying a medium blue color and a smooth surface. Turquoise from the United States or Mexico is opaque, ranging from light blue to bluish-green or teal. Chinese turquoise from Hubei is often sky blue, light blue, bluish-green, green, or pale with white streaks, spots, or black iron lines. Egyptian turquoise may display tiny round blue spots. The blue color of turquoise is influenced by the presence of copper and is also thought to be related to the organic compound ammonia and copper. The water content in turquoise affects its color, and dehydration can cause the blue color to shift towards green. The green color in turquoise is associated with the substitution of iron ions for copper and aluminum, with higher iron content resulting in yellow-green hues.

 

Optical and Fluorescent Properties:

Turquoise has a waxy to glassy luster and is semi-transparent to opaque. Its color can range from light to medium blue, bluish-green, teal, and green. It exhibits weak short-wave fluorescence and weak to no or very faint green-yellow to blue fluorescence under long-wave ultraviolet light. The absorption spectrum of turquoise sometimes shows two weak absorption bands at 432nm and 420nm, with the former being slightly stronger. Occasionally, a weak absorption band may be observed at 460nm.

Turquoise features vibrant colors and has been widely admired both in ancient and modern times. It has a delicate and soft texture, moderate hardness, and a beautiful color. However, there are notable variations in color, hardness, and quality. It can generally be categorized into four major types: porcelain turquoise, green turquoise, frothy turquoise, and iron-wire turquoise.

 

Turquoise is a mineral composed of phosphate and copper-aluminum minerals, characterized by its opaque sky-blue color. It can vary from light blue to blue, green, light green, yellow, greenish, gray, and white. It typically has a hardness of 5-6, a specific gravity of 2.6-2.9, and a refractive index of 1.62. When exposed to ultraviolet light, it exhibits fluorescence ranging from light green to blue.

 

Turquoise is an ancient gemstone with a history of thousands of years, cherished by people through various dynasties. Turquoise deposits in Sinai were discovered by the ancient Egyptians over 5,500 years ago. Egyptian emperors dispatched elite teams consisting of thousands to tens of thousands of people, including soldiers, to mine and extract turquoise. Archaeologists found turquoise in the tomb of an Egyptian emperor, indicating that turquoise was worn as early as 5500 BC.

 

In 1927, the renowned Chinese geologist Zhuang Hongzhao wrote in his book "Shi Ya": "This (meaning turquoise), resembles a pinecone, its color approaching the verdant green of a pine, hence its name." This is because its natural form often appears in nodules or spherical shapes, resembling the green color of pine, hence the name "turquoise."

 

The fractured surface exhibits a shell-like or granular appearance, with a Mohs hardness of 5 to 6. After weathering, the greenish-blue color of chrysocolla can turn white and is referred to as "white material." The hardness can decrease to around 3, and the density is approximately 2.76 g/cm³ (with slight variations). These properties vary depending on the degree of compactness; porous specimens tend to have lower hardness and density, resulting in a granular fracture.

 

Chrysocolla crystals are small and exhibit needle-like microcrystals ranging from 1 to 5 μm in size under a 3,000× electron microscope, giving them a delicate texture. High-quality polished chrysocolla surfaces can display a smooth, enamel-like finish. In contrast, lower-quality specimens are more porous and have a rougher texture.

 

(1) Structurally, chrysocolla can be categorized as follows:

 

① Chrysocolla with transparent crystals often occurs in small sizes and granular forms, typically yielding gemstones of no more than 1 carat.

 

② Blocky chrysocolla includes clumps, nodules, and dense blocks. It may have a yellow-brown or black-brown outer layer. Denser specimens with slightly higher hardness (H≥5) are polished to a porcelain-like luster, known as "porcelain chrysocolla," which is a primary material for jewelry and jade carvings. Less dense nodular varieties with lower hardness (below H<5) result from extensive weathering and can be used for sculpting.

 

③ White material refers to porous, low-quality chrysocolla with a white, airy appearance, often referred to as "bubble chrysocolla." It is not suitable for jewelry or carving and is mainly used as raw material for enhancement processes. It is artificially colored, waxed, and treated to imitate natural or synthetic chrysocolla.

 

④ Wire chrysocolla features coexistence with black iron and asphalt substances, resulting in black vein-like patterns on the surface of greenish-blue chrysocolla. This patterning typically does not significantly impact aesthetics and can still be used for jewelry.

 

(2) Geographically, chrysocolla from different regions varies in quality and appearance:

 

① Chrysocolla from Hubei, China, comes in sky blue, light blue, green, and moon white colors. It exhibits uniform color, dense structure, and porcelain or waxy luster. Mohs hardness ranges from 5.1 to 5.5, with a density of 2.2 to 2.85 g/cm³, generally around 2.7 g/cm³, indicating high-quality chrysocolla.

 

② Persian chrysocolla, found in Iran, displays medium to deep sky blue colors. It has small pores and fine texture with strong luster, and its density is slightly higher. Some varieties may have spiderweb-like lines and brown-black patterns.

 

③ Chrysocolla from the United States and Mexico varies widely in color, with good specimens exhibiting blue-green and green-blue colors, while lower-quality ones range from white to pale blue and have more pores and a looser texture. Generally, they require artificial treatment.

 

④ Egyptian chrysocolla appears in blue-green and yellow-green colors, often with dark blue spots. Its texture is relatively fine, but its color is less desirable.

 

⑤ Transparent chrysocolla from Virginia, USA, is a rare and valuable treasure among chrysocolla crystals, displaying a transparent sky-blue color.

 

⑥ Crystalline chrysocolla from the Liskeard area of Cornwall, United Kingdom, consists of small blue-green crystals and is found in mineral nests.

 

⑦ Iron chrysocolla from the United States, also known as "copper iron phosphate," has a chemical formula of CuFe3+6(PO4)4(OH)8 and contains significant iron content (up to around 5% Fe2O3). Its color is light, and its density is 3.1 g/cm³, with a refractive index ranging from 1.83 to 1.93, relatively high compared to other varieties.

 

⑧ Bone and tooth chrysocolla include bone chrysocolla and tooth chrysocolla. Bone chrysocolla forms from ancient animal bones and is partially replaced by iron phosphate (blue iron ore), resulting in beautiful blue colors. Extracted specimens often appear gray-blue and are commonly used as gemstones after heat treatment. Tooth chrysocolla forms when the teeth of large ancient mammals (such as ancient elephants) are replaced by iron. These types of chrysocolla are also artificially colored, often using heated sulfuric acid copper solutions to create imitations. Originating from locations like the Creuse department in France and northwest Siberia in Russia, these are sources of bone chrysocolla. Some of these chrysocolla specimens were excavated alongside blue iron ore at a construction site at Xi'an Jiaotong University in 1958.

 

 Identification Characteristics

 

Green iron-bearing chrysocolla resembles natural turquoise, but turquoise has lower density and refractive index. Due to the presence of chromium, turquoise appears pink when viewed through a Chelsea color filter, while chrysocolla, with its copper content, displays blue or green hues due to iron impurities.

 

Chrysocolla often contains brown limonite or small black pyrite-like inclusions, and some specimens have asphalt veins. In the gem trade, chrysocolla with a web-like inclusion pattern is referred to as "spiderweb chrysocolla." Yellow iron minerals and quartz inclusions are also common in chrysocolla from the United States.

 

 Comparison with Similar Minerals

 

(1) Differentiating from chrysocolla, chrysocolla has higher refractive index (N=1.50), lower density (2–2.5 g/cm³), and lower hardness (2–4).

 

(2) Compared to treated chrysocolla, artificially treated chrysocolla can be bleached by dropping a drop of ammonia benzene dye. Chrysocolla treated with oil or wax will show signs of melting when a hot needle is brought close, with flowing wax or oil visible under a magnifying glass. Chrysocolla injected with plastic may emit an unpleasant plastic odor during this test. The heat needle test should be brief (around 3 seconds) to prevent discoloration.

 

(3) Dyed jadeite with a similar appearance to chrysocolla can have lower refractive index (N=1.54) and sometimes exhibit a banded structure, appearing pink under a Chelsea color filter.

 

(4) Fossilized teeth and bone colored with iron phosphate are referred to as "tooth chrysocolla" in Japan. Natural substitutes for turquoise include iron copper phosphate, hydroxy borosilicate, and hydroxy aluminum sodium phosphate. These imitations have vastly different properties and appearance from genuine chrysocolla.

 

(5) Synthetic chrysocolla is uniformly sky blue and exhibits greenish spherical structures under 50× magnification. Differences between chrysocolla and similar stones are outlined in Table 13-3-1.

 

(6) Block chrysocolla is created by pressing small pieces or fragments of chrysocolla together, bonding them with epoxy resin, and then cutting, shaping, and polishing them into finished products like chrysocolla eggs and bracelets. Assembled fragments are easily recognizable. The infrared spectrum of chrysocolla is shown in Figure 13-40-3.

 

Grading

 

In the international gemstone industry, chrysocolla is classified into four grades based on color, texture, and block size.

 

(1) Grade 1 (Persian Grade): Vivid sky-blue color, uniform coloration, soft luster, absence of iron lines and cracks, slight transparency, smooth and fine surface, representing the highest quality. Persian turquoise, Hubei turquoise, and high-quality turquoise from the United States belong to this grade. Persian turquoise with spiderweb patterns is categorized as Grade 1B. Turquoise with varying degrees of iron lines is referred to as Grade 1C.

 

(2) Grade 2 (American Grade): Deep blue color, opaque, slightly subdued luster, less vibrant color, absence of iron lines (Grade 2A), fine iron-line spiderweb patterns (Grade 2B), or numerous iron lines in various forms (Grade 2C).

 

(3) Grade 3 (Egyptian Grade): Green or bluish-green color, blue spots, coarse or porous texture, numerous iron lines, lower quality.

 

(4) Grade 4 (Afghan Grade): Varies in shades of yellow-green, specimens with numerous iron lines are not used for gemstones.

 

There are also three levels of hardness for turquoise: "Porcelain Turquoise" (hardest, H=5–6), "Hard Turquoise" (H=4.5–5), and "Soft Turquoise" (softer, below 4), also known as "White Foam," which has the lowest value due to significant weathering and reduced hardness.

 

 Enhancement Treatments

 

Various enhancement treatments are applied to lower-quality turquoise to improve its appearance. These methods include:

 

(1) Wax Impregnation: Wax is applied to the surface to seal minor pores and enhance the turquoise's color. The wax can be melted with a hot needle, appearing as low-density regions under magnification. Prolonged heating or sunlight exposure can lead to fading.

 

(2) Filling Treatment: Colored or colorless plastics or epoxy resins with added colorants are injected to fill surface pores, enhancing stability and appearance. Such filled materials often have lower density (2–2.48 g/cm³), lower refractive index (<1.61), and lower Mohs hardness (3–4).

 

(3) Dyeing Treatment: Black liquid shoe polish and similar materials are used to mimic darker matrices. This treatment can be detected by dropping hydrochloric acid, causing rapid yellow-green discoloration in imitation turquoise.

 

Occurrence and Origin

 

Turquoise is primarily found in supergene enrichment deposits. These deposits are associated with weathered crusts containing phosphates and copper sulfides. The host rocks can be relatively recent acid magmatic rocks (such as rhyolites, andesites, quartz porphyries, monzonites, and granites) or phosphatic sedimentary rocks (sandstones, siltstones, shales, mudstones, etc.).

 

Turquoise is a regenerated gemstone mineral that forms in fractures within weathered igneous or sedimentary rocks. Its historical origin is in the Nishapur region of Khorasan Province, Iran, which has been a renowned source of high-quality turquoise historically referred to as "Persian turquoise." The Sinai Peninsula in Egypt is another important source of turquoise. Other known sources include Uzbekistan, northern Chile, Hubei and Shaanxi provinces in China, near Brisbane in Australia, Colorado and Arizona in the United States, and many others, all yielding relatively high-quality turquoise.

 

In China, Yun Gaosi deposit in Yun County, Hubei Province, is one of the main historical sources of turquoise. It is found in fractures within lower Cambrian black carbonaceous and siliceous shale and siliceous slate. The turquoise occurs in a lens-like or discontinuous vein-like filling in structural fractures. The best quality turquoise is sky-blue or deep blue, followed by grass-green or bluish-green specimens, often associated with limonite and kaolinite. Other Chinese sources include Zhushan County in Hubei, Yunxi County in Hubei, and areas extending from Baitushan in Shaanxi to Pingli. The Baitushan region in Shaanxi is particularly famous for producing large, high-quality, blue-colored, hard and compact turquoise, with a history of extraction dating back to the Qing Dynasty.

 

 Turquoise Care

 

Turquoise is delicate and sensitive to contamination. It should be protected from contact with tea, soap, oil, rust, and other substances that could seep into its pores and cause discoloration. During carving and crafting, a clean environment should be maintained to avoid soiling. Turquoise is sensitive to high temperatures and should not be exposed to open flames; excessive heat can lead to fading or cracking. Sunlight exposure can also cause fading and drying. Turquoise can lose its color when exposed to alcohol, aromatic oils, soap bubbles, and certain organic substances. Avoid contact with cosmetics and perfumes when wearing turquoise jewelry.

 

It's generally not recommended to use heavy liquids or refractive oils for density and refractive index testing, as some turquoise specimens are porous, which can affect the accuracy of the test.

 

[Appendix] Synthetics and Imitations

 

Due to its popularity, turquoise has various synthetic, treated, and imitation forms. Synthetic and imitation turquoise have been present since the 1970s. One notable synthetic turquoise is produced by Gilson, closely resembling natural turquoise in chemical composition, crystal structure, refractive index, and density. However, microscopic examination reveals tightly packed small spheres, unlike the scaly structure of natural turquoise. There is ongoing debate about whether these features distinguish genuine synthetics, as visual identification can be challenging.

 

Other common forms of imitations include:

 

(1) Reconstituted Turquoise: Low-quality turquoise (such as white foam) is ground into powder, mixed with resin, plastic, or glass, and dyed to resemble turquoise. This reconstituted turquoise exhibits reflections typical of resin, plastic, glass, or glaze and is denser and harder than natural turquoise.

 

(2) Stabilized Turquoise: Created using magnesite, calcite, dolomite, hematite, and synthetic colorants in 2004, often resembling turquoise in color but is an imitation.

 

(3) Turquoise Glass: A synthetic turquoise glass product made from quartz, copper compounds (possibly chrysocolla), calcium carbonate, and sodium carbonate. It has been used for carving since ancient Egypt and as a pigment.

 

(4) Turquoise Powder Composite: A 2004 introduction involving the compression of organic dyes and glue into magnesite, later refined to incorporate inorganic dyes.

 

(5) Ceramic Turquoise: Produced using ceramic methods, resulting in light blue to medium blue colors, often featuring a spiderweb-like pattern. It lacks absorption bands at 432nm and 420nm, present in natural turquoise.

 

 

 

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