I'm not sure about what you should seal it with, but for future reference here is some advice from another discussion list:
"When doing polished stone, if you use the Brilliance metallic inks you do not need to worry about sealing it and it tries just as quickly as the non-metallic colors you will use."
I followed her advice and have only used the Brilliance metallic reinkers, and I've had no problems. Hope that helps.

Linda