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Spinel

Spinel, traditionally considered as ‘semi-precious’, is a fine gem that is rapidly growing in popularity.  Spinel has a bright vitreous lustre and can be found in a wide array of colours including red, blue, violet, and pink.

Spinel - Semi-precious stone
Red spinels
 

Red and pink colours are a result of trace amounts of chromium (Cr) present in the structure while the blues are caused by small amounts of iron (Fe) or sometimes cobalt (Co). Spinel’s chemical composition is very close to that of corundum’s and that has led to confusion in the past (see Fun Fact section of this text).

Where can you find the spinel you need?

Pierres de Charmes carries a vast selection of fine quality spinel.  Our collection of well-cut stones includes many different shapes and sizes in a wide spectrum of colours. 

Spinel

Spinel

Réf. # 1365

Spinel

Spinel

Ref. # 1603

Spinel

Spinel

Ref. # 1604 - 8.57 x 7.53 mm - 2.63 ct

Spinel

Spinel

Réf. # 1605

Spinel

Spinel

Ref. # 1607 - 10,40 x 7,60 mm

Spinel

Spinel

Ref. # 1612 - 6,78 x 6,08 mm - 2,29 ct for the pair

Spinel

Spinel

Ref. # 1613 - 6,78 x 5,81 mm - 1,91 ct for the pair

Spinel

Spinel

Ref. # 1640 - 6,18 x 5,65 mm - 1,11 ct

Spinel

Spinel

Ref. # 1726 - 5,62 x 4,43 mm - 1,16 ct for the pair

Spinel

Spinel

Ref. # 2943 - 7,22 x 6,00 mm - 1.11 ct

Spinel

Spinel

Ref. # 3189 - 5,00 mm

Spinel

Spinel

Ref. # 3190 - 5,50 mm

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Spinel Fact Sheet

 

Spinel possesses all the qualities necessary for use in jewellery and its beauty has captivated gem connoisseurs for centuries.  With a hardness of 8 on the Mohs scale, a bright vitreous lustre, an excellent toughness and stability spinel is suitable for use in all types of jewellery.  Spinel can also show a colour change effect and occasionally display a four or six-rayed star. [1]

Chemical Composition MgAl2O4
Crystaline system Cubic
Cliveage No cliveage
Refractive index 1,712 à 1.740[2]
Birefrengence none
Isotropique  
Density 3.80 à 4.05

 

Gemmologists can use many instruments to identify spinel.  The presence of trace elements such as chromium and cobalt produce diagnostic absorption spectrums that can be easily detected using a portable instrument called a spectroscope. 

Using magnification (loupe or microscope) certain inclusions such as healing fissures or solid octahedral shaped crystals allow gemmologists to distinguish between natural and synthetic spinel. 

Producing Countries

 

Important spinel deposits are found in Myanmar, Sri Lanka, Afghanistan, and Australia.  Spinel is also found in Brazil, Nigeria, and the United States.  Spinel deposits are often in the same metamorphic or alluvial occurrences as corundum.

Rough spinel crystals generally present as more or less well-shaped octahedrons.  The crystal faces can be so perfectly smooth that they appear to have been polished.[3]

 

Is spinel treated?

 

Spinels are amongst the few gems that do not respond well to heat treatment.  Spinel is sometimes treated along with other gems such as ruby however; the treatment does not significantly improve the gem’s appearance.  Rare cases of colour improvement have been reported for spinel making it important to stay informed about new treatments as techniques evolve.

Some spinels have surface reaching fractures and like other gems with these inclusions they may be treated with an oiling process that fills the fractures to improve the stone’s appearance.  This treatment is not stable and Pierres de Charme does not buy or sell oiled spinel.

Spinel in Jewellery

 

Even though most consumers are unfamiliar with spinel, jewellers have long appreciated this gem for its incredible lustre and vast array of natural colours. Spinel’s excellent durability makes it an ideal choice for all types of setting and jewellery styles.  In August 2016 spinel joined peridot as an official birthstone for the month of August.

Value and Quality Criteria

As with most fine and semi-precious gems spinel’s quality and value depends largely on the saturation and intensity of its colour. Vibrant reds, a result of trace amount of chromium in the gem’s composition (ruby also owes its colour to this element), rich blues due to the presence of cobalt, and bright pinks and oranges are the most desirable colours.

Clarity will also have an influence on value. Spinel can show inclusions visible with the unaided eye or a jeweller’s loupe. Gems with less obvious inclusions will have a higher value than gems with inclusions that are easy to see.

The quality of the cut will influence the gem’s overall appearance and have an impact on value. A well-cut gem with proper proportions and symmetry will display more brilliance and be more beautiful than a poorly-cut stone. Given the rarity of spinel, many lapidaries must often choose between making a larger poorly-cut stone and a smaller well-cut stone. These larger stones maximize the yield from the rough crystals but because they are not cut with proper proportions they do not maximize the return of light and will show less brilliance.

The size and weight of a gem will also influence the price. Large uncut crystals of spinel are rare and therefore large facetted stones are also quite rare. There is a dramatic increase in per carat prices for fine quality red, blue and pink facetted spinels that weigh over 5 carats.[4]

All of the above quality criteria must be taken into consideration when establishing the value of spinel. The value may also be influenced by market trends and the gem’s country of origin.

Fun Fact

Historically, spinel was often mistaken for ruby.  Both gems are often found in the same deposits, they look alike, and they have similar chemical compositions; it was not until the 18th Century that spinel was correctly identified as a separate mineral.  As a result of this confusion, many famous “rubies” are in fact spinel.  Perhaps the most famous of these is the ‘Black Prince Ruby’, which is a 5 cm high red spinel, featured in the English Imperial State Crown. 

[1] (2010). Spi-1. In Cours de base en gemmologie Gem-A. London.

[2]  Gem property Chart A [Chart]. (n.d.). In Gemological Institute of America (GIA). 1992

[3] (2010). Spi-1. In Cours de base en gemmologie Gem-A. London.

[4] Schumann, W., Georges-Catroux, F., &Poirot, J. (2014). Spinelle. In Guide des pierres précieuses: Pierres fines et ornementales: 1900 échantillonsphotographiés (p. 116). Paris: Delachaux et Niestlé.