Last updated on August 3, 2023
Spinel is one of the three birthstones of August, along with peridot and sardonyx. Spinel is a rare gemstone that often appears flawless and comes in a range of beautiful colours. It is also the fourth most durable gemstone with a rating of 8 on the Mohs scale, which comes just behind sapphire, ruby and diamond.
Spinel Origin and Formation
Spinel is a captivating gemstone with a rich geological history. Its origin and formation involve intricate processes that take place deep within the Earth’s crust over millions of years.
Spinel forms under high pressure and intense heat and originates in the Earth’s mantle, which lies between the crust and the core. The mantle contains various minerals and elements, including magnesium, aluminum, and oxygen, which are essential components of spinel. Under extreme heat and pressure, these elements combine to create the mineral structure of spinel. Additionally, the presence of other trace elements can influence its colour, leading to a wide spectrum of vibrant hues.
Spinel is often found in association with certain types of rocks, such as metamorphic and igneous rocks. Metamorphic rocks are formed through the alteration of existing rocks due to heat and pressure within the Earth’s crust. When these rocks experience the right conditions, such as contact with hot magma or the tectonic movement of plates, spinel can be formed as a product of the metamorphic process.
Igneous rocks, on the other hand, are formed from the cooling and solidification of molten lava or magma. When the magma cools rapidly, it can trap spinel crystals within the rock structure.
Additionally, spinel can be found in alluvial deposits, areas where minerals and gemstones have been eroded from their original rock formations and accumulated in riverbeds, sediment, or gravel. Over time, natural processes like erosion and weathering carry spinel crystals from their sources to these alluvial deposits, where they are discovered and mined.
The regions where spinel is commonly found include India, Pakistan, Afghanistan, Myanmar (Burma), Tanzania, Sri Lanka, Australia, Russia, Vietnam, and other parts of the world with suitable geological conditions for its formation.
Spinel is renowned for its stunning array of colours, making it a highly sought-after gemstone for jewellery and collector’s pieces alike. The diverse range of spinel colours is a result of the presence of trace elements and impurities during its formation.
One of the most famous and coveted colours of spinel is a rich, vibrant red, often resembling the hue of rubies. These red spinels are affectionately known as “ruby spinels” or “traffic light red.” They owe their colour to the presence of chromium in their crystal structure. Spinels can also exhibit delicate and romantic shades of pink. These captivating hues arise from the combination of chromium and occasionally iron in the gemstone’s composition. Pink spinels can range from pale pastels to deeper, more saturated tones.
Some spinels display a warm and lively orange colour. The presence of chromium and a higher concentration of iron contributes to the development of these fiery shades. Blue spinels are relatively rare but highly prized. The blue colour comes from the presence of cobalt in the gem’s composition. Spinels with purple and violet hues owe their beauty to the combination of manganese and cobalt. These gemstones can display a range of shades, from soft lavender to deep violet.
Yellow and green spinels are relatively rare in nature; however, they do exist and can be highly prized when found. The presence of iron and varying oxidation states contribute to these unique colours. Spinels can also be found in cool and sophisticated shades of grey and black. These colours result from varying concentrations of manganese and iron.
One of the most enchanting properties of spinel is its ability to exhibit colour-changing characteristics under different lighting conditions. The colour change in spinel is typically observed between two distinct hues, often shifting between blue, violet, and purple in daylight to red, pink, or mauve under artificial or incandescent light. This remarkable shift in colour can be quite dramatic and adds a unique charm and desirability to the gemstone.
Spinel, as a gemstone, generally exhibits excellent clarity due to its crystalline structure and formation conditions. The occurrence of inclusions in spinel is relatively rare compared to other gemstones, such as emeralds or rubies, which often have more visible flaws. Spinel’s formation process, which involves high pressure and intense heat deep within the Earth’s crust, allows the gemstone to form with minimal internal disruptions.
The most common types of inclusions found in spinels are tiny rutile or needle-like crystals, which may be present due to the conditions under which the gemstone was formed. These inclusions are generally inconspicuous and do not significantly impact the stone’s clarity or beauty.
When evaluating the clarity of spinel, gemologists use a standardized grading system similar to other gemstones. The clarity scale ranges from “eye-clean” (no visible inclusions to the naked eye) to “included” (visible flaws). Most spinels fall into the “eye-clean” to “slightly included” categories, with only a small percentage of stones falling into the “included” range.
The cut of spinel is a crucial aspect that significantly influences its overall beauty, brilliance, and visual appeal. A well-executed cut can enhance the gemstone’s natural colour, maximize its sparkle, and highlight its unique features. The primary goal of cutting a spinel is to achieve the best possible combination of brilliance, clarity, and symmetry while preserving as much of the gem’s carat weight as possible.
The brilliant cut is the most common and popular cut for spinels. It consists of a series of triangular and kite-shaped facets that optimize light reflection and create exceptional brilliance. The step cut is often used for darker-coloured spinels, while the mixed cut is a versatile option that helps to highlight the gem’s colour while also enhancing its brilliance. The cabochon cut is often used for spinels with unique colour zoning or inclusions that can create interesting visual effects.
Spinel Carat Weight
Spinel comes in a wide range of sizes, from tiny gemstones measuring just a few millimetres to large and rare specimens exceeding several carats. Generally, as the carat weight of a spinel increases, so does its value, assuming other quality factors remain constant. Larger spinels are relatively rarer to find compared to smaller ones, making them more valuable and sought-after by collectors and jewellery enthusiasts.
The price per carat of spinels is not linear; larger stones command a higher price per carat than smaller ones. This is because larger spinels are scarce and have a higher perceived value. As a result, a 2-carat spinel might not cost twice as much as a 1-carat spinel of similar quality; it could be significantly more expensive.
While small spinels (under one carat) are relatively common and accessible, larger sizes, especially those exceeding 5 carats, are considered rare and highly valuable. Fine-quality spinels in larger sizes can command premium prices at auctions and high-end jewellery markets.
|Chemical Name||Magnesium aluminium oxide|
|Colours||Red, yellow, orange, brown, blue, violet, purple, pink, green, black, grey, colourless|
|Hardness||8 on the Mohs scale|
|Refractive Index||1.712 - 1.762|
|Specific Gravity||3.54 - 3.63|
|Transparency||Transparent to opaque|
|Fluorescence||Red spinel - strong red fluorescence, blue spinel - weak or no fluorescence|
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