Why is my ♦ hammer (i.e. diamond) so durable, but it can burn? For those confused about the title, there's a joke that every moderator has a "diamond hammer" (diamond from the little ♦ by the username). In a chat discussion, it was brought to my attention that a diamond can burn (which makes total sense because it's pure carbon) after I got a notification in orange in the topbar of SE.

Anyway my question is why can something so durable be able to burn? I would imagine that there would be a relationship between flammability and durability, especially since metal cannot burn.* Is it just a coincidence that most people think hard == inflammable? Why can diamond burn?

*It can burn when oxidized IIRC.
It is bond strength, not hardness, that determines how easily oxygen
  can attack and burn a material, allowing me to burn a diamond in a
  pool of liquid oxygen resting in a block of graphite.


Diamond is hard because its bonds form an inflexible, three-dimensional lattice. However, the strength of these bonds themselves is not even as strong as graphite, for example. Graphite has much stronger bonds, but they create "sheets," which can slide around, making graphite softer but harder to burn.

(source)

Indeed, people have burned diamonds before by putting them in tubes of pure oxygen.