Rubies will be found in veins identical to Emeralds, but the ore will be red instead of green. You can use Rubies to trade with Zombie Pigmen wich will sell things similiar to regular, vanilla minecraft Villagers. You'll get a better deal if the Zombie Pigmen are of higher levels. If they're angry, you can't trade with them.  Rubies can be found in the Nether, making them hard to spot in the Netherrack mass.