Corundum - Sapphire & Ruby

Sapphires Have a Surprisingly Large Range of Colours

Padparadscha - Orange Sapphire

Yellow Sapphire

An Attractive Cabouchon Sapphire

Large Oval Ruby

Colours
Corundum can be colourless, red, pink, blue, black, brown, orange, yellow, green, indigo, violet, or mauve. Red corundum and most pink corundum is called ruby, blue corundum is called sapphire, and other colours are also called sapphire, usually with the colour specified as a prefix to the word sapphire, for example, yellow sapphire.

Colouration
Pure corundum is colourless, often called white, and although quite rare, tends not to be valuable because it does not have much brilliance. Colours, as in many gemstones, are caused by small amounts of impurity, usually metallic oxides. This is a case where impurity is desirable.
Chromic oxide causes brilliant red colouring in corundum, thereby producing rubies.
Ferric oxide causes yellow colouration, titanium oxide produces vivid blue.
In fact the colouration of corundum is not quite so simple as this. The titanium and iron are usually present in the form of ilmenite, a mineral which is a titanium iron oxide, TiFeO₃. Ilmenite is not isomorphous with aluminium oxide. Isomorphous means being able to replace the host mineral within its crystal structure. Instead ilmenite is present as a microscopic inclusion, in the form of colloidal particles.
This colloidal nature may be responsible for other optical effects such as "silk", asterism, and colour banding.

Hardness
Corundum is very hard, having a hardness of 9 on Moh's scale, compared with 10 for diamond, and 8 for topaz. Hardness is generally a desirable feature is gemstones.
Other uses for corundum, because of its hardness, are as watch bearings, watch glasses, and as an abrasive.

Chemical Composition and Name	Al2O3 - Aluminium Oxide
Hardness	9
Refractive Index	1.759 - 1.767 to 1.770 - 1.779
Specific Gravity	3.96 - 4.01
Birefringence	0.009
Crystalline System	Trigonal

Ruby

Padparadscha

Sapphire