Slate is fine but marble is not. MArble is too soft and will leach into your water.
Granite is also good.
If after a few minutes it starts to foam, then don't use it in the tank as this is an indication that the rock will slowly dissolve in your tank. Also avoid any rocks that appear to have any veins of metal running through them. The safest rocks for a freshwater tank are lava rock, slate, quartz, onyx, and pertrified wood. Do not trust a rock not to leach or dissolve simply because the petstore told you it was safe. If you doubt the safety of the rock do not use it.