Just start from "dead rock". Because unless someone is breaking down a tank, it is unlikely you will get purple, coralline encrusted rock.
Also, frankly, people usually break down tanks when they are in a down cycle (due to aiptasia, algae overgrowth, etc).
Only "good time" is if someone is downsizing tank and has extra rock.
Even then, they usually out it in a big container and run water through it without light, so you lose the purple coralline algae anyways.