How do I restore the black color of the slate.

I have a nineteenth-century slate topped table.  Whatever finish it has
on it makes it susceptible to water stains.  Any water left on the table
top turns it white.  How do I restore the black color of the slate where
it has been marked by water?  How do I put a finish on the top that will
make it resistant to being stained by water.  Any help would be much
appreciated.