The term 'sapphire' is not used for which colour of corundum stone?
Corundum is a crystalline form of aluminium oxide (Al2O3). It can have different colours depending on the presence of transition metals (iron, titanium, venadium, chromium) as impurities. Corundum has two gemstone varities: Sapphires and Ruby.
Sapphire occurs naturally in its crstalline alfa-A2O3. It is typically blue in colour, but other colours are: yellow, orange, green, brown, purple and violet hues. Particoloured 'sapphires' are those which exhibit two or more colours in a single stone. These 'sapphires' cannot be created synthetically and only occur naturally.
The term 'sapphire' is not used for red colour, as it is called a 'ruby'. Ruby is corundum stone which contains chromium impurities (hence red colour).
Blue 'sapphires' with upto 15% violet or purple are generally of fine quality. Padparadscha 'sapphires' are rare. It is originally found in Sri Lanka, and also found in deposits in Vietnam and parts of East Africa. Corundum that contains >0.01% of titanium is colourless. The natural 'sapphires' are cut and polished into gemstones and worn in jewellery. The 'sapphires' are also used in infra-red optical components, wristwatch crystals and high-durability windows.