Garnet is a fusion — i.e., two Gems combining personalities and appearances as one shared holographic body — formed by two Gems named Ruby and Sapphire, who choose to remain permanently fused out of love for each other.

What Gems make Garnet?

Garnet is the fusion of Ruby and Sapphire and the current de-facto leader of the Crystal Gems.

What gem does ruby and Sapphire make?

Out of pure joy and pride, Sapphire jumps into Ruby's arms and the two accidentally form Garnet, blowing the Crystal Gems' cover.