The lesser-sung cousins of corundum like sapphires and rubies, spinels are resilient gemstones that come in a variety of colours from greys and purples to ‘Jedi’ red. Here are some quick geological facts to get you started with this beautiful gem material.
Note: Don’t get confused with synthetic (lab grown) spinel, commonly coloured blue with cobalt.
Nature
Chemical composition: Magnesium aluminium oxide
Structure: Cubic crystal system, isotropic optical properties
Crystal habit: Octahedral, macle, ‘spinel twins’
Durability: No cleavage, good to excellent toughness, very good stability, very good hardness (8 on the Mohs scale equivalent to topaz)
Colour: Wide variety of hues especially red, pink, orange, blue, purple, grey, black, also colourless.
Inclusions: Minute octahedral crystals of spinel-type minerals, Zircon haloes (esp specimens from Sri Lanka), iron-stained fractures.
History, culture, identity
Historically mistaken for ruby and sometimes referred to as ‘balas ruby’. Spinel and ruby are often found in gem deposits together. Famous examples are found in royal regalia e.g. Black Prince’s ruby (red spinel) in the Imperial State Crown of the British Crown Jewels. It is estimated to weigh 170 carats.
Spinel, especially from Afghanistan and Pakistan, may still be marketed as ‘Kashmir ruby’. The specimen I photographed below was sold to me as such. In Myanmar (Burma) spinel is called ‘Nat Thwe’ meaning ‘polished by the spirits’. Recently spinel was declared a birthstone for August, alongside Peridot by the American Gem Trade Association (AGTA) and Jewelers of America (JA).
Origin
Spinel crystals usually form in metamorphic rocks, those that have undergone chemical changes because of massive heat and or pressure over time, but without melting. Spinel crystals are often found at sites of contact metamorphism, such as in a marble rock matrix. Marble is limestone that has metamorphosed. Though not gem quality, spinel may also form part of polycrystalline igneous rocks.
Occurrence
Spinel when found and extracted as gem material occurs in placer alluvial deposits (secondary deposits). These are deposits found in active and ancient river beds where denser, more durable gem materials have collected and concentrated. Rough spinel is often found with corundum (ruby and sapphire) in gem gravels. Gem crystals can also be extracted from their host rock (primary deposit).
Localities
Alluvial deposits (secondary)
- Sri Lanka (Zircon haloes often seen as inclusions)
- Myanmar (Burma)
Mined crystals (primary)
- Afghanistan
- Pakistan
- Australia
- Brazil
- Nigeria
- USA
