Here is an old thread of mine on Dendroboard that touches on semi precious stone/mineral/metal safety (and possibilities!) a bit...
http://www.dendroboard.com/forum/par...struction.html

But one thing you can do with just about anything... is spray it down in clear plasti dip, or clear spray polyurethane (generally food safe in most forms once cured), both should be inert non toxic coatings when dry and which waterproof the material and prevent leaching of anything harmful.

Slate should work fine though.