Spinel is one of the three birthstones of August along with peridot and sardonyx. Being a rare gemstone that often appears flawless, spinel comes in a range of beautiful colours. It is the fourth most durable gemstone with a rating of 8 which comes just behind sapphire, ruby and diamond.
Spinel Origin and Formation
Like rubies and sapphires, spinels are formed under immense pressure and intense heat under the earth’s surface for millions of years. Moreover, they are usually found in the same areas which explains why they are often confused with each other.
Spinels consist of magnesium, aluminum and oxygen atoms which turn into liquid when hot magma emerges from below the earth’s surface and seep into cracks in igneous or metamorphic rocks.
When the liquid cools it turns into colourless crystals. However, this rarely happens in nature as magnesium aluminum oxide mixes with impurities and gets different colours.
It is worth mentioning that spinel is most commonly mined as an alluvial deposit. This means that before being discovered and collected, spinel has been eroded from the rock in which it formed.
Spinels can be found in India, Pakistan, Afghanistan, Myanmar, Tanzania, Sri Lanka, Australia, Russia and Vietnam.
The wide colour range of spinels is caused by impurities or miniscule traces of other minerals mix added during the birth and growth process.
If the microelement is chromium then the spinel turns red, pink or orange. In the case of iron, we get deep red colour spinels that look very similar to rubies.
Blue, purple, violet, grey and black spinels are caused by various combinations of manganese and cobalt microelements.
Yellow and green spinel is extremely rare in nature but it can be easily created in labs so be wary when you see such colours in shops.
The most popular colour in spinels is ruby red which is sometimes referred to as “traffic light red”. However, as naturally blue gemstones are rare the cobalt blue is probably the most sought after spinel colour.
Neon pink spinels from Mahenge, Tanzania are also one of the most desired gemstones due to their amazing natural colour.
The colour changing phenomenon can also be seen in spinels. One type is bluish-grey in daylight and light purple under artificial light, others turn from pink to lilac and blue to purple.
To figure out which spinel colour is the best, we need to consider the tonal grade and saturation of the stone.
Tone refers to the level of lightness or darkness of the colour. It is graded from very light to very dark where medium tone spinels are the most valuable.
Saturation refers to the intensity of the colour. This characteristic defines if the gem has weak, vivid, strong or intense colouring. For example, some red spinels may look brownish, while blue, purple and violet gems may look a little grey. Spinels with strong saturation have the nicest deep colouring.
Spinel Clarity and Cut
Natural spinels can be either transparent, translucent or opaque. However, in some cases, all three can be seen in one gemstone.
Even rough natural spinels are very transparent which allows seeing any internal blemishes and inclusions so the clearer the gem the better. However, distinctive fingerprint-like inclusions may add to the gemstone’s charm.
Since spinel is a durable gemstone (8 on the Mohs scale) it can be cut and faceted into almost any shape but cutters need to take special care to enhance the brilliance and maximize scintillation.
Since spinel is quite a rare and valuable gemstone it can often be cut into a non-standard shape and size to minimize the rough yield.
The price of natural spinels is determined by their colour, clarity, cut and size.
Since deep red and cobalt blue spinels are the most popular, they command the highest prices. Pink, violet, orange, grey and black varieties can be found at much more reasonable prices.
In general, spinels with no inclusions or flaws visible to the naked eye are more valuable than those with obvious impurities. Although, the gems with star effect which is caused by inclusions can be rather attractive.
Larger spinels over 5 carats in any colour other than black are very rare, meaning they are very expensive.
|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||1.71 - 1.73|
|Specific Gravity||3.54 ~ 3.63|
|Transparency||Transparent to opaque|
|Fluorescence||Red spinel - strong red, blue spinel - weak reddish, green|