There are many ranges and varieties of coloured gems. The main ranges of popular coloured gems are corundum, rubies and garnet.
Corundum
Corundum is a crystalline form of aluminium oxide typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral of A12O3. It is naturally transparent material, but can have different colors depending on the presence of transition metal impurities in its crystalline structures
Corundum family consists of red ruby and all sapphires, (except red sapphires). It has a MOH hardness of 9.
Corundum or generally coloured stone with nice medium tone has better value than light or dark tone colours.
Corundum are usually rated based on hue (colour), tone, saturation or the intensity or purity of colour and the distribution of colour on the piece of jewel.
Rubies
Rubies are considered one of the most valuable gemstone. Rubies are harder and comes with a much more brilliant red and is much more expensive, compared to red garnet.
Rubies have the moh hardness of 9. Rubies are vivid red though sometimes they have purplishor bluish secondary hues. Hold your stone up to a bright light source and move it around until it creates a rainbow. Due to the nature of rubies, they absorb greens and yellow in the color spectrum, hence if they are rubies, you will not see green and yellow in the rainbow spectrum.
Garnet
Almandine is an iron alumina garnet, of deep red colour with tinge of black or reddish brown. The moh hardness for garnet is 7.5 to 8.5. Pyrope garnet is usually dark, blood red colour and is a very attractive gem.
Orange Sapphire Padparadscha
Recently, popular in the market is the padparadscha sapphire, which mainly comes from Sri Lanka. It is bright orange coloured sapphire. There are many other ranges of sapphires. Sapphires are corundum and has mohs scale of 9.