What is a Sapphire?

For those curious as to the science and the gemmology of sapphire this article is a great guide.

What is in it?

Sapphire is a gemstone that is mainly composed of corundum along with; iron, titanium, chromium and copper. All of which are minerals. If you see sapphire that is of a blue, yellow, purple, orange or even a greenish colour, minerals such as iron, titanium, chromium copper and/or magnesium give the corundum that colour.

What is Corundum?

Corundum is a crystalline[1] form of aluminium oxide. The crystalline has traces of other minerals such as iron, titanium and chromium[2]. In its most natural state, the mineral is transparent. This does not remain true because the corundum can actually shift colours when tainted. The transparent minerals are used as gems and are called ruby if they give a red glow and padparadscha[3] if given a colour of pink-orange. All other corundum that projects any other colour are labelled as sapphire.

There are two dominant ways in which corundum is formed. One is the metamorphosis[4] of limestone and the other involves the igneous occurrence in rocks lacking silica[5].

Corundum is rated as a 9.0 out of ten in terms of the Mohs scale of mineral hardness.

What is Aluminium Oxide?

Aluminium Oxide consists of the compounds oxygen and aluminium (Al2O3). Alpha-Al2O3 or corundum is the most commonly known and significant modifications of aluminium oxide.

Aluminium oxide is produced by machine from the mineral bauxite. There are currently 20 billion tons of bauxite deposits in the world. Aluminium and aluminium oxide are manufactured by the principles of the Bayer method: Bauxite[6] is broken and dried and is then dissolved using concentrated sodium hydroxide soloution. The left overs (iron, silicon and titanium) are segregated from the bauxite in the red mud. The aluminium hydroxide is then precipitated from the mix and then calcinated[7] at 200-1300°C to form Al2O3.

How are Sapphires Cut?

Sapphires are very rare gems. You will often see sapphire presented as jewellery in the shape of either an oval or a cushion. Ovals and cushion tend to preserve most of the original rough. Round sapphires are popular as well, however their final shape involves removing a large portion of rough and are as a result are costlier than ovals and cushions.

Sapphires are treated by several methods so that the quality of the gem and colour are enhanced. One technique of doing this is by heating. This type of treatment is done in furnaces between 500-1800 degrees Celsius for hours at a time. By heating the sapphire, the colour of the gem becomes more blue. As well the stone becomes more clear because it loses silk and it becomes clearer under magnification. This is just one of other techniques used to treat sapphire.

Where is Sapphire Mined?

Sapphires are found in igneous[8] and metamorphic rocks. When sapphires are extracted from solid host rock they are labelled as primary deposits. They are considered as secondary deposits when they are found some distance from their original source. Rocks are hit against by water and wind. The process of erosion moves small pieces of rocks into streams of water where they are broken, releasing any gems that they contain.

Word Search


[1] Pertaining to crystals or their formation.

[2] A lustrous, hard, brittle, metallic element used in alloy steels for hardness and corrosion resistance.

[3] A delicate light to medium toned pink-orange to orange-pink hue corundum.

[4] Any complete change in appearance, character, circumstances, etc.

[5] The dioxide form of silicon.

[6] An amorphous clayey rock that is the chief commercial ore of aluminum.

[7] To convert into calx (the oxide or ashy substance that remains after metals, minerals, etc., have been thoroughly roasted or burned) by heating or burning.

[8] Geology . produced under conditions involving intense heat, as rocks of volcanic origin or rocks crystallized from molten magma.



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