Part of what's interesting is the pure geekish coolness of how granite works. Part of it is underlining the idea that nature doesn't care about purity. Granite forms the major part of the continental crust. Crystals growing into their "perfect" shape is *not* the default.
It's natural for cultures to influence each other, and human societies generally consist of many cultures entangled with and impinging on each other.
Granite isn't perfectly similar to many-cultures societies (if it were, it wouldn't be a metaphor)--it doesn't convey the way cultures are changing each other, or that individuals have a fair amount of choice, but I think it goes some distance towards conveying the idea that you can't drastically simplify a society without damaging it.