Blue Diamond, or Blue for short, is a Homeworld Gem and a member of the Great Diamond Authority that formerly ruled over the Gem race. She first appears in a flashback in "The Answer" and debuts in person in "Steven's Dream".

As a result of Era 3, Blue Diamond now uses her powers to bring joy to other Gems through her clouds, created from her vaporized tears, which make someone happy once coming into contact with them.