Slate is inert. Wash it first!
All of my slate (something like 250kgs) comes from garden centre stock. Not from a fish store! In fact I highly recommend garden centres for feature substrate pieces as it works out MUCH MUCH cheaper!
They usually have a good range of rock too, so, if you know what is available in a fish store, and you see some in a garden centre, buy it!
The only thing is garden centre stuff is a little dirtier, but that's all.
Green wood will rot, even dry wood rots or breaks down. Bog wood goes through a process underground which prevents this occurring.
Wiki covers bog wood!