Its quite likely to work well; couple of caveats tho:
Test the shale with a coupla drops of muriatic (sp?) acid; if it bubbles don't use it. It will mess up the water parameters.
Boil the hell out of it, or soak it in bleach to remove any possible microscopic hitchikers. Definitely don't want to add that to your tank as well!
aka Cycling Guru and the Ich Slayer
*glares at Terry and QTOFFER*
Card carrying member of FTAS & GCAS.