People value turquoise for its combination of ancient heritage and attractive color. Fine turquoise played a major role in civilizations past and present. Here are some samples that show the use of turquoise in the pre-historic time.
Turquoise is found in dry and barren regions where acidic, copper-rich groundwater seeps downward and reacts with minerals that contain phosphorous and aluminum. It’s judged on three basic qualities—its color, texture, and the presence or absence of matrix.
The most-prized turquoise color has to display an even, intense, medium blue shade. Some consumers prefer a greenish blue, and some contemporary designers actively seek avocado and lime green turquoise.
Turquoise most highly valued color is an intense, evenly distributed, medium blue known as Persian blue in the trade.
The traditional source for the top color, sometimes described as robin’s-egg blue or sky blue, is the Nishapur district of Iran, the country formerly known as Persia. So, quite often, you’ll hear people in the trade call turquoise of this beautiful color as “Persian blue,” whether or not it was actually mined in Iran. Although Persian blue is favored, the turquoise market also places high value on other shades of blue. As with Persian blue, these shades are often associated with a traditional source. For example, American or Mexican turquoise is light blue, or greenish blue to bluish green, while Egyptian turquoise is greenish blue to yellowish green.
Scientists believe that the reason some turquoise looks greenish blue is because it contains more iron than pure-blue turquoise. The bluer shades are caused by greater amounts of copper. Any greenish color tends to lower turquoise value. In any turquoise hue, a uniform and even distribution of color is preferable to mottled or uneven distribution. Splotches of white or green in a blue stone, for example, make it less valuable than one with uniform blue color.
Turquoise body-color can be unstable particularly in an untreated, light-blue cabochon, for example, might gradually darken and discolor over time. In the past, people who believed turquoise had supernatural qualities thought that its tendency to discolor might represent the stone’s sympathetic response to its wearer’s mood. Today, gemologists know that discoloration usually happens in reaction to contaminants in the environment.
The turquoise’s degree of porosity influences how much its color changes over time. The more porous the turquoise, the more likely it is to absorb foreign substances that can spoil its color.
Turquoise owes its texture to its composition. It’s usually a cryptocrystalline aggregate of microscopic crystals that form a solid mass. If the crystals are close together, the material is less porous, so it has a finer texture. Fine-textured turquoise has an attractive, waxy luster when it’s polished. On the other hand, turquoise with higher porosity, which means a coarser texture, has a dull luster after it’s polished. Porosity and texture don’t just affect appearance: They also affect durability. Turquoise with coarse tend to be fairly soft—it ranks 5 to 6 on the Mohs scale and might have poor toughness, too. Low porosity and fine texture are more valuable in turquoise than high porosity and coarsetexture.
Turquoise deposits usually form in limonite or sandstone. Limonite matrix creates dark brown markings in turquoise, while sandstone creates tan markings. The desirability of matrix depends on the market and on the balance and attractiveness of the matrix markings.
Colorado Gold Nugget And Bisbee Turquoise Ring by John Hartman
Manufacturers try to fashion turquoise so that no matrix is visible, but sometimes it’s unavoidable. Small amounts of turquoise might be scattered through the host rock in such a way that the rough material can’t yield any cut specimens large enough to fashion into gems without including some matrix. Just as the presence of matrix lowers jadeite value, it also affects the value of turquoise.
Bisbee turquoise with sandstone matrix-photo by Roger Weller
Some buyers actually prefer the presence of matrix in fashioned turquoise if its effect is attractive and balanced. This is especially true if it’s a type of turquoise known in the trade as spiderweb turquoise. It contains matrix in thin, delicate, web-like patterns across the face of the gemstone. The patterns provide a dark contrast to the gem’s bright blue. In the market for top-quality turquoise, stones with no matrix at all command the highest prices. Gems with attractive spiderweb matrix rank second in value. Matrix-free and spider web turquoise command the highest prices.
TREATMENTS IN TURQUOISE
The supply of fine quality turquoise is limited. Treatments can make lower-quality turquoise more stable and attractive. Treatments like stabilizing can make turquoise color more stable by sealing out foreign substances like cosmetics, perspiration, and grease.
Treatments in turquoise include:
In America, ancient civilizations inhabiting what are now Mexico and Central America sent traders north along trade routes to Cerillos, a no-longer-active turquoise mining region in the Sangre de Cristo Mountains near Taos, New Mexico. This remote area supplied societies to the south, including the ancient Mayans, with the gemstone they prized more highly than gold. In fact, a Navajo origin legend tells that the first man and woman created the sun from a stone disk edged with turquoise.
Ancient Persia was also a bountiful source of turquoise. But the ancient mines of Persia and New Mexico no longer yield significant quantities of turquoise. Today’s sources include the US, China, Egypt, Chile, and Iran. Because turquoise typically found in association with copper deposits, turquoise is occasionally a byproduct of copper mining. This is the case in open-pit copper mines in Arizona.
In the 1960s and 1970s, the US state of Nevada’s Lander Blue mine produced a beautiful dark blue turquoise with a black spider-web matrix pattern.The material was wildly popular, but the mine closed after producing only about 100 pounds of rough.
Lander Blue - turquoise is considered the most valuable turquoise blue in the world and has become the most popular amongst turquoise collectors.
Kingman mine- has been producing turquoise from the copper mineral exploration area in the Mineral Park Mining Region in State of Phoenix ( AZ ). The color ranging form light blue to very dark blue usually with a white matrix and frequently flecked with pyrite mineral. High-grade Kingman has always been considered among the top quality turquoise.
Kingman Turquoise with tan colored matrix of sandstone
China’s Hubai Province- is the world’s major source of top-color, sky blue turquoise.
The southwestern US produces the largest quantities of turquoise. The Sleeping Beauty mine in Globe, Arizona, is probably the largest turquoise mine in the United States.
Turquoise imitations made of glass and plastic are fairly common.
Plastic is especially prevalent. The plastic commonly sold in art supply stores takes a polish similar to turquoise and is easier to work with than the real thing. It’s also much less expensive. The temptation to deceive can be high, and the result is convincing, so buyers of turquoise jewelry should try to be sure that their purchases aren’t set with plastic imitations.
Another type of imitation is reconstructed turquoise. The name is misleading because this imitation often contains no turquoise at all. This type of material is actually a mixture of powdered minerals dyed and bonded with plastic or epoxy resin.
In addition to manmade imitations, a few natural materials can resemble turquoise enough to serve as look-alike alternatives.
Howlite, a semitranslucent to opaque mineral, is white with a dark gray or black spiderweb matrix.When the material is dyed blue, it resembles turquoise. Howlite is found in the US in limited quantities.
Variscite is a translucent to opaque mineral found in the US that ranges from light to medium yellowish green to bluish green. It so closely resembles turquoise in color that it’s sometimes known as Nevada or California turquoise. Variscite’s occasional yellow to brown matrix markings heighten its resemblance to turquoise. The stone’s availability is limited.
A blue variety of chalcedony, colored by the mineral chrysocolla, can also strongly resemble turquoise.
It is known as chrysocolla chalcedony. Translucent to semitranslucent, this US gem is an intense light blue to blue green. And like variscite and howlite, its availability is limited.