Granite is harder than slate (hardness of 7 vs. 51/2) but that isn't the whole story. Hardness is resistance to scratching (not an issue for pool table beds). Toughness (resistance to breaking) is what you're after and slate wins in that department. In the world of rocks slate is often referred to as "flexible". The moisture thing you're talking about is an issue, but I don't hear many pool players complaining about that. The carom guys are very concerned with that, which is why they prefer heated tables......with thick slate beds for durability.