Captain Shatner wrote:
"For some strange, unknowable reason, dilithium is impossible to replicate. Whether this be the instability of the nucleus or because Kirk said so, you just can't make dilithium."
Considering dilithium crystals are still a form of matter and are in close proximity where the annihilation of matter and antimatter take place, it's doesn't seem to be that strange
to me that it's something you can't replicate (like antimatter). The properties of dilithium appear somewhat supernatural.
If this were the hardest natural substance known to federation science (cast rodinium is or was at one point the hardest artificial substance) the question would be inevitably how you could be capable to give it a specific shape or form.