The color of rocks and minerals can provide important clues about their composition and origins. While many rocks exhibit more common colors like gray, brown, or red, some rocks and minerals display the more unusual combination of blue and green hues.
Rock Types with Blue and Green Colors
There are a few rock varieties that commonly exhibit vivid blue, green, or blue-green colors:
- Chrysocolla – A copper-based mineral that forms vibrant blue and green gemstones.
- Amazonite – A green variety of microcline feldspar.
- Turquoise – A BLUE-GREEN hydrated phosphate mineral.
- Malachite – A GREEN copper carbonate mineral.
- Azurite – A DEEP BLUE copper carbonate mineral.
- Aventurine – A variety of quartz with glistening inclusions that produce a light blue-green color.
Of these options, the rock most well known for its distinctive blue-green color is turquoise. Turquoise has been prized as a gemstone for thousands of years and has a striking color ranging from sky blue to greenish robin’s egg blue.
What is Turquoise?
Turquoise is an opaque, BLUE-GREEN mineral that has been used in jewelry and other ornamentations for thousands of years. It has been especially prized in Persia, the Southwestern United States, and Central America.
The characteristic color of turquoise comes from its composition. Turquoise is a hydrated phosphate mineral with the chemical formula CuAl6(PO4)4(OH)8·4H2O. It forms when copper-rich solutions interacts with phosphate and aluminum-rich rocks.
In addition to its vivid BLUE-GREEN color, turquoise is also distinct among minerals for its waxy to dull luster. The hardness of turquoise on the Mohs scale is 5 to 6, so it can be scratched by a knife blade but not by many other common materials.
Where Does Turquoise Form?
Turquoise forms in copper-bearing rock when groundwater circulates through fractures and voids in the rock. The water oxidizes the copper, producing blue-green colored compounds.
Turquoise deposits are found in arid climates within igneous or sedimentary rocks that have some copper content. Major turquoise mining regions include:
- Southwestern United States (Arizona, New Mexico, California, Nevada)
- Sinai Peninsula in Egypt
- Northern India
- Northern Chile
- China
- Cornwall, England
The southwestern United States produces the finest quality turquoise. Important localities in this region include the Cerrillos Hills and Burro Mountains in New Mexico and Morenci, Bisbee, and Kingman in Arizona.
Uses of Turquoise
The vibrant BLUE-GREEN color and ornamental quality of turquoise has made it highly desirable for jewelry, art, and decorations since ancient times. Significant uses include:
- Jewelry – Turquoise has been used for bracelets, necklaces, rings, brooches, and other jewelry for thousands of years.
- Art – Turquoise figures prominently in the art of many Southwestern Native American cultures.
- Decoration – Turquoise has been used to adorn weapons, ceramics, sculpture, and many other decorative objects.
- Carvings – Intricate carvings have been fashioned out of solid turquoise.
- Mosaics – Turquoise chips and tiles have been used to create mosaics and inlays.
- Talismans – Many cultures have used turquoise as a protective amulet due to its blue-green coloring.
This wide array of uses has made turquoise one of the most valued opaque gemstones. Vivid blue specimens with no matrix are the most highly prized.
The Value of Turquoise
Natural turquoise varies widely in quality, so values span a broad range. High quality turquoise is valued at $40-$100 per carat. The most valuable turquoise exceeds $1,500 per carat and can rival fine diamonds in price.
Factors that influence the value of natural turquoise include:
- Color – Bright blue and greenish blue colors are most valued. Green and grayish blue colors are less desirable.
- Matrix – Little or no veining and spotting increases value. Heavily matrixed stones are worth much less.
- Clarity – Opaque, non-chalky material is preferred. Translucent and chalky turquoise is less valuable.
- Cut – Finely cut cabochons or natural water-worn nuggets fetch higher prices.
- Carat Weight – Larger stones are exponentially more valuable.
- Provenance – Turquoise from certain localities like Arizona and Persia is more prestigious.
With so many factors influencing price, it takes an experienced jeweler to properly appraise quality turquoise specimens.
| Turquoise Value Factors | Low Value | High Value |
|---|---|---|
| Color | Greenish or grayish blue | Vivid blue |
| Matrix | Heavily matrixed | Little or no matrix |
| Clarity | Chalky | Opaque and non-chalky |
| Cut | Poorly cut cabochons | Finely cut cabs or natural nuggets |
| Carat Weight | Under 5 carats | Over 20 carats |
| Provenance | Unknown source | Arizona or Persia |
Imitation and Synthetic Turquoise
With natural turquoise reaching such high values, imitation and synthetic materials are commonplace. These turquoise simulants include:
- Reconstituted Turquoise – Natural turquoise shards and dust that are bonded with resin.
- Dyed Howlite – WHITE howlite soaked in BLUE-GREEN dye.
- Dyed Magnesite – Dyed GREEN or turquoise blue.
- Dyed Jasper – Jasper dyed turquoise blue.
- Plastic – Molded plastic beads colored turquoise blue.
- Synthetic Gilson Turquoise – Laboratory-grown turquoise.
These imitations may resemble turquoise, but they lack the beautiful veining patterns and depth of color found in natural specimens. Identifying fakes requires a trained eye and lab testing in difficult cases.
Notable Turquoise Specimens
Some record-breaking turquoise specimens include:
- The Smithsonian Turquoise – A 12.5 pound, apple-sized turquoise from Arizona, valued at $1.15 million.
- The Candelaria Turquoise – A perfectly colored 10 pound nugget discovered in Nevada in 1887.
- The Pilot Mountain Turquoise – A 445 pound chunk unearthed in Nevada in 1993, valued at $125,000.
Large natural nuggets like these are exceptionally rare. More commonly, turquoise is recovered in small veins, nodules, and fracture fillings within host rock.
Conclusion
With its striking BLUE and GREEN coloration, turquoise stands out among gemstones. Natural turquoise has been prized for thousands of years, but varies widely in quality. The finest turquoise can reach prices comparable to diamonds. Turquoise has enduring popularity for jewelry and decorative objects in many cultures worldwide.