SPOILERS FOR ATTACK ON TITAN, READ AT YOUR OWN RISK...
Hardening, or Titan hardening is an ability posessed by certain types of titans in the anime Attack on Titan. Since the setting has rifles and cannons, hardening comes in handy to protect against them.
Now, I'm not trying to imitate the ridiculous speeds at which titans harden themselves, only their toughness. R... the Armoured Titan tanked several direct cannon shots and put a hole in a giant wall via body slam, its armor plates sustaining little visible damage in the process.
So, since I love stealing concepts from other people, I wanted to create something similar. You see, hardness equals bullet resistance (except for diamond, diamond is crash). Sure, after the first hit, ceramics lose a lot from their defensive value, so combining them with nicely oriented fibers is the best way to make them last.
The thing is, I don't know what the hardest thing is that a biological process could plausibly produce. Note that we're talking about Vickers hardness here.
What's the hardest substance a biological process can produce?