Rubies are used for Enhancing weapons and armor. Rubies can be obtained as random drops from monsters, or via a Blacksmith NPC for the price of 50,000G. All Rubies do the same thing.
Unlike Emeralds or Sapphires which each have specific upgrade options, all Rubies are the same and consequently all sell for a similar amount. Rubies can be sold to an NPC for 15,000G, however, it is common for people to vend Rubies for approximately 30,000G.