Corundum, colour, clarity and cut are the four Cs in this instance.
(Corundum; image courtesy of Geology.com)
Corundum is the mineral that births rubies and sapphires and is a rock forming mineral known for its quality of hardness (placing the ruby third in line after the diamond) 9.0 on the Mohs scale. Dating back to antiquity, the ruby is considered one of the most precious gemstones; in the company of the diamond, emerald, sapphire and amethyst. It is red in colour, hence the name ruby which comes from the Latin word ruber which stands for the colour red. The ruby gets its colour from the mineral known as Chromium because its compounds can be vividly coloured. Chromium, whose name, incidentally comes from the Greek word chroma which means colour.
(L to R: A pigeon-blood red ruby; image courtesy of Pinterest and a pink sapphire; image courtesy of James Allen.)
The most significant part of a ruby is the shade of red it holds. Blood red or Pigeon blood fetches the highest price and the lightest shade being a light pinkish colour can classify it as a pink sapphire, a separate gem altogether (which is debatable). Orange, pink and purple are also hues that can be showcased by a ruby.
(Silk inclusions occurring in a ruby stone; image courtesy of GIA edu.)
Next in line is the clarity of the stone. Rubies tend to have inclusions (a characteristic within the stone itself, visible due to the transparency of the stone) and this may diminish the value of the gem but the complete absence of the inclusion can also be taken to mean that the gem has been treated.
(The Star Effect in a ruby, en cabochon cut; image courtesy of GIA edu.)
As far as the cut goes, en cabochon (smooth and not faceted) rubies are typically the ones that visible asterism which is the star effect. Transparent rubies have a glassy lustre. Rubies that have rutile inclusions (as explained above) have a silky shine; like star rubies.
(L to R: The Sunrise Ruby; image courtesy of Sotheby’s and Jim Stein, owner of Stuart Kingston Jewelers, poses with a custom case holding the Liberty Ruby; image courtesy of The Daily Mail.)
The Burmese rubies are well-renowned gems the world over, making Burma (Myanmar) a significantly important source for this particular gemstone. Sri Lanka and Thailand are important sources of the ruby as well, among other countries in the world.
Some famous rubies include the Sunrise Ruby, the Liberty Bell Ruby, the Rosser Reeves Ruby, the DeLong Star Ruby and the Hope Ruby among others.