Turquoise is a uniquely colored mineral that has been prized for thousands of years. Its striking blue-green hue sets it apart from other gems and gives it an instantly recognizable look. But where exactly does turquoise fall on the spectrum of mineral colors and classifications? Here we will explore the technical definition and classification of turquoise to understand its place among colored minerals.
The Composition and Structure of Turquoise
Turquoise is considered a hydrated phosphate mineral with the chemical formula CuAl6(PO4)4(OH)8·4H2O. It forms in rather specific environmental conditions when water percolates through aluminum-rich rocks containing copper, typically in arid climates. The copper gives turquoise its distinctive blue-green color while the aluminum and phosphorus provide its structure.
Turquoise most often occurs as cryptocrystalline masses that show no obvious crystal structure. However, small submicroscopic crystals can sometimes be discerned under magnification. These crystals belong to the triclinic crystal system, meaning the unit cell is defined by three unequal axes that are not perpendicular to each other.
Classification by Color
One of the primary ways turquoise is classified scientifically is by its range of colors. While turquoise is typically thought of as a blue-green stone, it can vary quite a bit in both hue and saturation. Here are the major color varieties of turquoise:
| Color Variety | Description |
|---|---|
| Blue turquoise | Ranging from sky blue to greenish-blue, this variety usually has high iron content |
| Green turquoise | Apple green to yellow-green hue, caused by higher copper content |
| Yellow turquoise | Butter yellow or wax yellow, rare color caused by dehydration |
| Brown turquoise | Dull brown or tan, caused by dehydration |
| White turquoise | Chalky white color, caused by complete dehydration |
As the table shows, the color of turquoise depends heavily on its hydration state and relative copper to iron content. The most valued shades are a vivid robin’s egg blue to a deep sky blue.
Hardness Classification
In addition to color variety, turquoise can be classified according to hardness. On the Mohs scale of mineral hardness, turquoise generally falls between 5 to 6. The Mohs scale ranks minerals on a scale from 1 (talc) to 10 (diamond) according to their relative ability to resist scratching.
Here is how the hardness of turquoise can be categorized:
– Soft turquoise: Mohs 5, can be easily scratched by a knife
– Hard turquoise: Mohs 6, can be scratched by a steel point
– Extra-hard turquoise: Mohs 7 to 8, resists scratching and has dense matrix
The softer grades of turquoise are more fragile and chalky while the harder varieties are suitable for jewelry and ornamental objects. Geological factors like the matrix composition affect the overall hardness.
Matrix Patterns
Turquoise is also classified based on the matrix pattern seen when it is cut or polished. The matrix refers to the vein-like webbing of rock that runs through turquoise. It may consist of granite, quartz, pyrite, or other associated minerals. Here are the main matrix patterns used to describe turquoise:
| Matrix Pattern | Description |
|---|---|
| Spiderweb | Delicate veining resembling a spider’s web |
| Turquoise in matrix | Large turquoise areas set in dark matrix background |
| Birdseye | Round turquoise spots resembling a bird’s eye |
| Waterweb | Irregular matrix veining resembling water |
| China matrix | Thin black veining resembling Chinese porcelain |
The most valuable patterns have either minimal or delicately lacy matrix veining. A solid color without matrix is also highly prized.
Geographic Source and Color
Turquoise is also classified by its source location, which impacts the color shades. For example:
– Iranian turquoise: Sky to robin’s egg blue
– Turquoise from the American Southwest: Blue to light green
– Chinese turquoise: Vivid sky blue to azure blue
Certain mines became famous for producing signature colors of turquoise. Examples include the Sleeping Beauty mine in Arizona, known for rich blue turquoise, and the Kingman mine in Arizona which produced intense blue-greens.
While most turquoise is still identified by its source location, scientific advances now allow origin determination using spectroscopic and chemical analysis. This provides definitive proof of a turquoise sample’s geographic origin.
Natural vs. Treated Turquoise
Another criterion used to classify turquoise is whether it is untreated or has undergone enhancement. Completely natural turquoise with no enhancements is the most valuable type.
However, the majority of turquoise used in jewelry today is treated or enhanced in some way. Common turquoise treatments include:
| Treatment Type | Description |
|---|---|
| Waxing | Penetrating wax to improve color and luster |
| Dyeing | Penetrating stain to change color |
| Reconstitution | Binding fragments with epoxy resin |
| Stabilization | Hardening with plastic or resin |
| Oiling | Surface coating with mineral oil |
So a high quality natural turquoise with no treatments is becoming increasingly rare and costly. Most commercial turquoise will have some level of enhancement, which should be disclosed when selling it.
Grading Systems for Turquoise
There are a few standard grading systems used to assess and classify turquoise specimens:
The AAA Quality Rating:
– AAA – Highest quality turquoise, outstanding color and pattern
– AA – Good color and matrix patterns
– A – Average quality in terms of color and matrix
The Durango System:
– A Grade – Natural, untreated turquoise
– B Grade – Color enhanced turquoise
– C Grade – Reconstituted turquoise from fragments
– D Grade – Imitation turquoise made of plastic or other materials
The Cut Quality Grading:
– Excellent – Superior cut with no visible matrix
– Good – Minor visible matrix or flaws
– Fair – Obvious matrix or flaw areas
– Poor – Prominent matrix or flaws
These rating systems cover the important factors like color, matrix patterns, and quality of cutting or manufacturing. They provide standardized ways of judging turquoise quality.
Valuation and Pricing
The classification criteria above all influence the valuation and pricing of turquoise. Here are some of the key factors that impact turquoise value:
– Color – Deeper sky blues and blue-greens are more valuable. Yellow, brown and pale turquoise is cheaper.
– Clarity – Clean, pure color without visible matrix is preferred. Heavily included stones are cheaper.
– Hardness – Hardness above Mohs 6 commands higher prices. Soft, chalky turquoise is cheaper.
– Cut quality – Higher grade cutting and polish raises value.
– Carat weight – Larger stones are exponentially more valuable.
– Treatments – Untreated natural turquoise is most costly. Treated stones are cheaper.
– Provenance – Famous locality turquoise like Sleeping Beauty commands higher prices.
– Rarity – Unique patterns or colors increases value. Common varieties are inexpensive.
So in summary, deep blue turquoise with no matrix and no enhancements will be the most valuable and expensive, assuming it also has good carat weight. The price goes down drastically for low grades of cut, off-colors, heavy matrix, and treatments. But even cheap turquoise has its appeal due to the remarkable signature color that makes turquoise instantly recognizable.
Conclusion
Turquoise is classified scientifically by its chemical composition and crystal structure, which place it as a hydrous phosphate mineral. But gemologically, turquoise is categorized according to physical properties like color, matrix patterns, hardness, cut quality and geographic origin. These factors all influence the valuation and pricing, with deep sky blue turquoise in large sizes commanding premium values. While enhanced turquoise is commonplace, untreated natural stones with rich color are the most prized. Whether sky blue or yellow-green, turquoise remains one of the most iconic and coveted colored gemstones due to its striking blue-green palette.