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  •  Gemstone Knowledge   •  The Sapphire – Gemological Facts
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The Sapphire – Gemological Facts

Gemstone Knowledge

The sapphire – Gemological Facts

What is a corundum? 

The sapphire is a variety of the mineral corundum the second hardest natural mineral known to mankind. Ruby is the red variety of corundum – all other colours are called sapphire, even pink. This means sapphires and rubies are both are varieties of the mineral corundum just in different colours.

What natural sapphire colours are available?

When you think of sapphires, you probably think of a rich blue colour, but sapphires actually come in almost every colour of the rainbow – including pink, peach, purple, yellow, orange, green, teal or colourless. Red sapphires are better known as rubies. Red sapphires are called rubies, depending on culture and continent there are differences between pink sapphire and ruby.

How do sapphires get their colour?

Sapphires get their colours from trace elements in the mineral corundum. Classic blue sapphires contain iron and titanium, and trace elements of chromium can turn corundum pink, while more chromium turns it into a ruby.

What is the hardness of a sapphire?

Sapphires are among the most durable naturally occurring elements in the world. Gemstones are rated on their ability to withstand scratching. Based on the Mohs Scale of Hardness sapphires score a 9 out of 10 only a diamond has more hardness but not such magnificent colours.

How rare are sapphires?

Every sapphires are a scarce natural resource. In particular very transparent stones with good clarity and colour distribution for faceting jewels. In terms of comparing same quality sapphires of up to one carats are more common, larger sapphires of up to three carats are possible to source but even more scare. However over five carats sapphires are very scarce and above ten carats sapphires are a exceptional jewel as an absolutely rarity.

At what temperature are sapphires heated?

Sapphires melt under very intense heat of around 2050° Celsius (or 3722° Fahrenheit). In comparison, silver burns at 962.8°C and gold at 1064°C. High temperature heating of sapphires take place around 1.700 to 1.900° Celsius.

What is pleochroism in gemstones?

Why is the viewing angle decisive for the sapphire color? Sapphires are “pleochroism stones”. Meaning, viewing these stones from two different angles (like the top view and the side view), may produce two different colours. Blue sapphires often have greenish blue and violetish blue pleochroism. It’s most desirable to orient the cut so the stone shows the violetish blue colour when it is set in jewellery. 

Since when are sapphire syntheses available? Since when are sapphires heat treated?

Sapphire is one of the very first gemstones to ever be flux grown in a laboratory (1902). Therefore the age of the sapphire do not give an indication whether it is natural or not. Same accounts for heat treatment of corundum which was firstly mentioned in 1240 AD.

Can sapphires change colour? Why are there colour changing sapphires? 

Some sapphires are known to have a colour change (similar to alexandrite), changing from blues hues in daylight to violetish blue, violetish purple or reddish purples in incandescent light. This effect is caused by the interaction of the sapphire, which absorbs specific wavelengths of light, and the light-source, whose spectral output varies depending upon the illuminate. Traces of metal impurities in the sapphire, such as chromium and vanadium, are responsible for the colour change.

What are star sapphires? How does the star get on the sapphire? 

Sapphires can have a starburst effect in them due to rutile or white silk, giving them the name “star sapphires“ based on the optical effect of asterism. The six-ray star pattern on the surface of the stone is only visible in cabochon-cut sapphire. Same applies of more intense pink and red corundum called “star ruby”.

Are there two-tone sapphires? Why are there sapphires with two colours?

Very rarely found are bi-colour sapphires. The bi-colour sapphire is a stone that contains two colours as a result of colour zoning. Colour zoning occurs when there is an uneven distribution of colour in a stone, and is caused during formation of the crystal when conditions of the trace elements which colour the stone, change.

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