The Ruby is red-colored, and the second rarest gemstone in the game, after Diamonds. It is used as an integral component of several items (stated in the infobox), some of which are used in other items (such as the Gold Crown being an ingredient of the Slime Crown).
A fast way to find rubies is to use the Spelunker Potion, which makes underground ores and gems shine. Another fast, yet foolish way, is to have bottles of Holy Water/Unholy Water or the Clentaminator and throw them in the area you are spleunking in. Gems will not change to Pearlstone or the other types of stone when in a biome. Although almost always seen underground, like other gems, there is a possibility that it may spawn in the overworld.
As with all of the gems, selling them is a good way to make some money.