Thanks, Jay!
It's Marshall Island Rock (expesive but really worth it in terms of the life that came on it - I've got at least 12 Astrea snails, vermetid snails, bunches of feather duster worms, caulpera, and unfortunately, green bubble algae :funny: )
I got it at a LFS - paid more than I would have on line but I was able to watch the rocks for a little over two weeks, pick out the pieces I wanted - hence the arch - and transport them home in water. All of which made the price worth it. An added advantage was that the tank went through its cycle in a little under three days.
I know that's blasphemy on this board

but I did have a Ammonia spike, followed by a teeny, tiny bit of Nitrite and Nitrate and the readings have been zero for over a week now :dance: It also allowed me to support a LFS, which I felt very good about.
K.M.