Subject: What exactly is orchalcum?

While researching Atlantis I came across descriptions of the alleged super 
society. It mentions that the walls around the city were covered by brass, 
tin, and orchalcum...which was discribed as(at the time)more valuable than 
gold. As for why..I'm attempting to write my first work of science 
fiction...and the bane of so many felines of course...couriousity.(please 
forgive the senseless beating of the english language at my can't-spell-
worth-a-tick club)

Re: What exactly is orchalcum?