Ruby is the most valuable and most precious gemstone used in jewelry. Untreated rubies of excellent quality are the most valuable gemstones on the planet. The second most precious gemstones are untreated emeralds. Diamonds are not most valuable and most expensive gems, unlike what most people think.
Ruby is crystallized aluminum oxide and is not a silicate unlike some very important gemstones like emerald and aquamarine that are silicates.
Red color of ruby is because of presence of chromium in its chemical composition. Pure crystalline aluminum oxide is colorless.
Nearly all rubies found in market are heat treated or fracture filled to improve color, clarity and look. It is extremely hard to find untreated high quality rubies. Heat treatment affects rutile inclusions present in rubies and sapphires.
Hardness of ruby on Mohs scale is the very excellent number of 9 and it is extremely resistant to scratches.