Ranz, if this is valonia, you should be able to roll a larger bubble off of the rock. Take it out of the tank and pop it. If it's a hollow "bubble", it is most likely valonia. If it isn't hollow, it's something else. I have some and it isn't black, but rather green.
Here is a pic..
http://www.globaldialog.com/~jrice/a...ge/valonia.htm