Maybe that's because of the biome you've chosen? I don't know another reason why it should do this. EDIT: I just did /p reset, and it unclaimed my plot. I'm sorry, but I think I can't help you any further with this.
I think it's to mark a plot is free to claim. I tried doing it, it turned to quartz but when I claimed it again it turned back to stone
Oh, okay. I think it would make sense to make all unclaimed plots quartz so they could be identified, but they only look like that after someone has claimed and unclaimed it.
^ Yes that would make sense, but this feature was only recently added, so only new unclaimed plots are affected.