I have an iPhone X and the glass is broken on the back of the phone. The screen is fine. Is this covered under warranty

No. The limited warranty does not cover any accidental damage. Also, Apple does not repair the rear glass and only offers full replacements at the pricing listed here:

http://support.apple.com/iphone/repair/service/#otherfees

warranty don't cover accidental damage only production defects only discovered alter on after purchase

Legal - Hardware Warranties - Apple