Ruby

Rubies are red gems, and are socketable items in Diablo II and Diablo III.

Diablo II
Like all other gems, Rubies grant bonuses that are related to their color. The red in Rubies signify the Elemental power of Fire. Socketing a ruby in Weaponry adds Fire damage, while Shields earn Fire resistance, and in Helms and Body Armor it increases life, which, in-game, is also associated with the color red. Crafted Items can be made with Rubies, and Blood Items have bonuses centered on increasing the wearer's life and recovering life.

Weapons: Adds Fire Damage

Shields: Adds Resistance to Fire

Helms and Body Armor: Adds to Maximum Life

Development
Prior to patch 2.3, Ruby gems in Diablo III had many more tiers, but were condensed for easier crafting.

Trivia

 * The name "Ruby" is derived from the Greek word for "blood:" i.e. "Rubinus." Its etymology as a "blood stone" is reflected in game, in which the Ruby is associated with an increase to life.