September 04, 2015Sapphires
The Colours of Sapphire
Did you know that sapphire started its journey as a colourless mineral? During its formation in the Earth, the basic mineral corundum* was exposed to tiny amounts of other elements such as iron and titanium. These elements became incorporated into the crystal structure. This changed the basic colourless crystals into the attractive colours we find today.
The term sapphire refers to all colours of the mineral corundum except the red colour. This colour has the name of ruby. The sapphire colours, other than blue, are known as fancy colours and include yellow, orange, purple and white, sometimes known as colourless.
When you think of a sapphire, it is the blue hue that often comes to mind. This is the colour that was sought by traders in the Roman empire, the jewellers of Byzantium, during the journey of Marco Polo and by British royalty. Blue sapphires were the perfect companion for diamonds in 19th century floral cluster and half hoop rings. Sapphire teamed naturally with diamonds in Art Deco white gold and platinum engagement rings.
Pink sapphire is highly prized for its attractive colour. The colour can range from a delicate pink to a shade with a slightly violet overtone. Larger stones are normal and like all sapphires, the pink stones have a high lustre.
When looking for unusual sapphire engagement rings
, the vibrant yellow and golden yellow colours are ones to consider. What makes the colourless corundum into a yellow sapphire? It is the presence of iron oxide alone. Historically, this attractive fancy colour was known as 'oriental topaz'. These yellow stones have great lustre and transparency and can often be seen in oval and rectangular step-cut shapes.
Most natural sapphires are heat treated to permanently enhance their colour and clarity. This is a practice that goes back to Roman times. Un-heated stones are quite rare and are substantially more expensive than heated stones.
Sapphire, unlike diamond, is dichroic, in other words, depending on how one looks at a gem stone, there will appear to be two different colours. In the case of blue sapphire, the dichroic colours are blue and violet although some blue sapphires show green and blue. Colour concentration in sapphire is rarely uniform throughout the crystal and is often more concentrated in one area of the gem than in another. It is the skill of the gem cutter that produces the fine gems that are used in sapphire jewellery.
* Corundum is aluminium oxide (Al2O3).