Board Thread:Theories/@comment-27441457-20151225042055/@comment-25322628-20160607013003

It's my belief that the Armored Titan's armor is made of hardened or dense bone, not as dense as probably the Female Titan's crystallization ability, but still strong enough to endure any attack directed at him.

Since diamond is viewed as the strongest substance in history, the Homelanders likely found a way to generate organic diamond as a trait for Titan Shifters.

It's also possible that the durability of each shifter's hardening ability is based on their experience and skill level with their Titan forms.

That's why Eren was able to do more with his hardening than Annie or Reiner.