The below picture was taken a few months after installation. See the rust on the chair rail and grout? The slate chair rail in this shower completely changed in color and now has a rusty appearance.
Some (usually darker-colored) slate varieties may have iron deposits, which can oxidize when exposed to water. Always avoid these stones altogether in wet areas. There is debate about whether certain types of sealers will stop this, but this is a trial-and-error approach, which is never recommended.
Note: Natural Stone names are private-labeled, and the quarry may use a different name than the supplier or retailer, so for this reason, we are not naming which stones to avoid, as it may not apply to a stone with the same name across the industry.