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by gagan choudhary

Turquoise: Blue of the sky & green of the sea


Gagan Choudhary, FGA


Part 1: Natural Turquoise

This is first part of the 3-part series on turquoise, covering its origin, formation, properties and care advice.

Turquoise’s sky-blue colour is one of the most popular colours not only in the world of jewellery and fashion, but also amongst the consumers referring to a specific shade of blue. Turquoise, a hydroxyl aluminium phosphate mineral was one of the earliest known gemstones used for adornment. It was used by the ancient Egyptian rulers since 5500 BC. In many cultures, it has been esteemed as a holy stone – a bringer of good fortune and/or a talisman. Turquoise is also the national gemstone of Tibet and has long been considered as a stone that brings good health, fortune, and protection from evil. Turquoise was also a ceremonial stone and a medium of exchange for Native Americans tribes in Southwestern America. Turquoise is a birthstone for December too!


The term ‘turquoise’ is derived from French expression, “pierre tourques”, meaning, “Turkish Stone”, due to the fact that turquoise first entered the western Europe through Turkey, and was coined around 13th century.

COLOUR

Turquoise ranges from blue to green, however, the most prized colour is even and intense blue of medium saturation, also referred as 'sky-blue' turquoise in the trade. Traditional source for this colour has been Nishapur in Iran, and hence this colour is also known as "Persian blue", irrespective of whether the stone is mined in Iran or not. Purer and more saturated the blue, higher the value. Demand of specific colour also depends on the consumer preference, as some prefer a greenish blue turquoise over a pure blue, while some prefer a lime green colour. Blue colour is caused by copper (Cu2+), while green due to ferric iron (Fe3+).

CLARITY AND PATTERNS

Turquoise is semi-translucent to opaque, with or without black to brown veins, known as matrix. These dark veins are remnants of the surrounding rock (or matrix) of turquoise and may cause interesting webbing pattern on the surface. A turquoise displaying distinct webbing pattern is popularly known as 'spider-web' turquoise. While a popular type of turquoise present in the trade is 'sleeping beauty' turquoise, which is known for its even colour (i.e., without any dark vein) and a sky-blue colour. The name 'sleeping beauty’ is after the mine name located in Arizona, USA where such material is mined. Sky blue turquoise without any matrix fetches the highest value, while presence of black veins or matrix lowers the value. However, turquoise with attractive spiderweb matrix ranks next to pure blue stones.

Figure 1: High-quality “Persian-blue” turquoise.

Figure 2: Greenish blue turquoise cabochons without matrix.

Figure 3: Blue and green ‘Spider-web’ turquoise

















Figure 4: Collection of evenly coloured beads of “Sleeping beauty” turquoise.

FORMATION AND STRUCTURE

Turquoise is formed in dry, barren and copper-rich regions, where rainwater or groundwater dissolves the copper present in the rocks to form an acidic solution; this copper-rich solution reacts with the phosphorous and aluminium present in the surrounding rocks to form semitranslucent to opaque compounds, called turquoise. As a result of this sedimentary process, the turquoise contains microscopic crystals forming a solid mass. If the crystals are packed closely together, the turquoise is less porous, and hence a finer and smoother texture. Turquoise with loosely packed crystals has higher porousity with coarser texture.


Often turquoise is found intergrown with other copper minerals, such as, malachite, azurite, chrysocolla, or other non-copper minerals like pyrite (also a common inclusion in turquoise) and quartz (seen as colourless to white grains).

Figure 5: Blue to greenish blue turquoise with metallic pyrite.





















Figure 6: Rough sky blue turquoise with grains of pyrite displaying dull lustre.

Both porousity and texture affect the appearance of turquoise. Lesser the porousity, smoother the texture, and vice versa. Fine textured turquoise has an attractive waxy to sub-vitreous lustre when polished, while a coarse textured turquoise appears dull after polish. Therefore, turquoise with low porousity and finer texture commands higher price compared to that with high porousity.

DURABILITY

The degree of porousity and texture also affects overall durability of turquoise. Stones with lower degrees of porousity and finer texture have better toughness against the stones which are more porous having coarser texture. Coarser turquoise appears ‘chalky’ when mined and are required to be stabilized with resins / polymers to make them smoother, shinier, and usable for jewellery.

PROPERTIES
Click to edit table header
Composition
CuAl6(PO4)4 (OH)8.5H2O
                Refractive Index
1.610 to 1.650
                Lustre
Dull to Waxy and Sub-vitreous
                Specific Gravity
2.35 to 2.90 (depending on degree of porousity)
                Hardness
  5 to 6

WHERE TURQUOISE IS FOUND

Iran

Turquoise has been mined in Nishapur district of Iran for more than 1000 years and is considered as a premium source. The prized turquoise with even blue colouration from Iran is referred as “sky blue” or “robin’s egg blue” or “Persian blue” in the trade – the terms used to describe the colour, even if the stone does not originate from Iran.


United States of America

Most of the turquoise originates from Arizona and Nevada, however, New Mexico was once the largest producer of turquoise, until 1920s. Arizona’s Sleeping Beauty mine was one of the most popular producers until a few decades back, but the stones from this mine are still available in the market.


China

Today, China is the World’s largest producer of turquoise. Hubei in Central China is the source for most of gem-quality turquoise being mined here.


TURQUOISE CARE

Since turquoise is a porous stone, it can easily be attacked or damaged by acids, cosmetics or even skin oils and perspiration. On exposure to these solvents, turquoise can easily lose its polish and lustre. Therefore, it is important to keep turquoise jewellery away from all types of solvents. However, it is safe to clean turquoise with warm soapy water but should be dried immediately with a soft cloth. It should not be exposed to steam or ultrasonic cleaners.


Hardness of turquoise is only around 5 to 6 on Mohs scale, therefore it is prone to scratches when stored with harder gemstones or other jewellery items. It is recommended to store turquoise jewellery separately.


Turquoise is usually stable to light and heat in routine wearing conditions; however, it should not be exposed to high heat, which can cause discolouration and breakage.


Turquoise might lack the brilliance, transparency, or clarity of other coloured gemstones like ruby, sapphire or emerald, but is a prime example of an opaque coloured stone that is popular as a gemstone for jewellery as well as an ornamental material. The light blue colour of the sky and lively green of the sea are such unique colours that a term specifically has been coined for it: turquoise.