You can cure cement into live rock, however a piece buried in the garden could have all kinds of crap leached into it, I personally would pass. Use all live rock (expensive) or you can also buy dry rock from a lot of the online vendors (less expensive)
Purple up is a sham product I would never buy (my personal opinion only) all you have to do is keep your calcium, alk and Ph in line and have proper light to grow it.
I'm fairly certain it is illegal to collect rock on any US coast, and to be honest the life you will get locally is not tropical and will likely die off in a tropical tank. In California you have tons of rock coming in daily, just find a store to sell you a piece that is a pound or two, and put it in the tank with the dead rock or home made rock. In a few months you will be sick of scraping coralline off the glass and will wonder why you ever wanted it so bad.