Corundum
The Science and Beauty Behind Ruby and Sapphire
Few gemstones command as much admiration as ruby and sapphire. Their vibrant colours and remarkable durability have made them favourites in jewellery for centuries. Yet both stones share the same mineral identity: corundum. Understanding what corundum is, how it forms, and the treatments it may undergo provides valuable insight into the fascinating world behind these treasured gems.
By Craig Arthur Thomas
Different trace
elements and combinations can produce sapphires in nearly every colour including yellow, pink, green,
and violet.
WHAT IS CORUNDUM?
Corundum is a naturally occurring mineral composed of aluminium oxide (Al₂O₃). In its pure form, corundum is completely colourless. However, most people encounter it in its coloured varieties: the red gemstone known as ruby and the many hues of sapphire.
The colours seen in these gems are not produced by aluminium or oxygen themselves. Instead, they come from trace elements, tiny impurities within the crystal structure. Because its colour depends on these external elements, corundum is described as allochromatic.
For example:
■ The vivid red colour of ruby comes from trace amounts of
chromium (Cr).
■ The classic blue of sapphire is created by small quantities of iron (Fe²*) and titanium (Ti4*).
COMMON TREATMENTS IN CORUNDUM
Many rubies and sapphires available in the jewellery market have undergone treatments to improve their colour or clarity. Some treatments are widely accepted in the gem trade, while others can significantly affect value.
Heat Treatment
The most common method involves heating corundum to temperatures between approximately 1,200°C and over 1,600°C. This process can dissolve rutile inclusions, enhance colour, and improve clarity. Because it has been used for centuries, it is generally considered a traditional and acceptable treatment.
Flux Healing
This process involves heating the gemstone in the presence of a chemical flux that partially melts material along fractures. As the stone cools, the material recrystallises, effectively “healing” cracks and improving both durability and appearance.
Diffusion Treatment
In this treatment, corundum is heated together with colouring elements such as titanium and iron (blue), chromium (pink to red), or nickel (yellow). These elements penetrate only a shallow layer beneath the surface, meaning colour may appear concentrated along facet edges or junctions, especially after repolishing.
Beryllium Diffusion (Lattice Diffusion)
This advanced form of diffusion uses beryllium to alter or intensify colour throughout the gemstone’s lattice. It can produce rare shades, such as the sought-after padparadscha-like orange-pink hues, or enhance existing colours in rubies and sapphires.
Fracture or Cavity Filling (Lead Glass Filling)
Lower-quality stones may be treated by filling cracks or cavities with glass to improve transparency and appearance. However, this treatment is less stable—over time, the glass can degrade, and exposure to chemicals or high heat may damage the stone.
Dyeing
Colour may also be enhanced using pigments suspended in oils, water, or filling materials. Gemologists can often detect dye residues within fractures inside the stone.
NATURAL AND SYNTHETIC CORUNDUM
Corundum used in jewellery can be either natural or synthetic. Natural corundum forms within the Earth under specific geological conditions and is generally classified according to its environment of formation. The two primary environments are:
■ Magmatic environments, where crystals form from molten rock
■ Metamorphic environments, where heat and pressure alter existing rocks to create new mineral structures
Synthetic corundum, by contrast, is produced in laboratories. One of the most common manufacturing methods is the Verneuil process, also known as flame fusion. In this process, aluminium oxide powder is melted at temperatures of around 2,000°C and then crystallised to form high-purity corundum.
Other techniques include:
■ The Czochralski method, where crystals are grown by pulling them from molten material
■ Flux growth, where crystals slowly develop from a molten chemical solution
These laboratory-grown stones share the same chemical composition and optical properties as natural corundum. As a result, distinguishing between natural and synthetic gems often requires specialised gemmological testing.
Different trace elements and combinations can produce sapphires in nearly every colour except red, including yellow, pink, green, and violet.
DURABILITY: A GEM BUILT TO LAST
Corundum is renowned for its exceptional durability, making it particularly well suited for jewellery worn every day.
Durability is determined by three key factors:
■ Hardness – resistance to scratching
■ Toughness – resistance to chipping or breaking when struck
■ Stability – resistance to chemical damage from acids or other substances
Corundum ranks extremely high in all three categories. With a hardness of 9 on the Mohs scale, it is second only to diamond in scratch resistance among commonly used gemstones. This combination of strength and resilience ensures that rubies and sapphires can retain their beauty for generations.
WHY ORIGIN AND TREATMENT MATTER
The origin of a gemstone – whether natural or synthetic – and any treatments it has undergone all play an important role in determining its value. Two sapphires may appear similar at first glance, yet differ significantly in price depending on these factors. For buyers and collectors, understanding these characteristics is essential. Knowing what you are purchasing ensures transparency in the marketplace and helps guarantee that the gemstone you choose truly reflects its worth.
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