There are a few possibilities, depending on what version you're thinking of:
* The song "Diamonds" by Rihanna: This version was written by Sia Furler, Benjamin Levin, and Mikkel S. Eriksen.
* The song "Diamonds" by Kanye West: This version was written by Kanye West, Malik Yusef, and No I.D.
* The song "Diamonds" by David Lee Roth: This version was written by David Lee Roth and Ted Templeman.
It's important to specify which version you're interested in to know who wrote it.