jws91 you are correct in saying ome Grantes are more porous than others.
But... I can't say it enough every Stone should be sealed regardless how hard it is. Not just for porosity but for other reasons also. All stone breath whats in the air, in time the stones will change colour if they are not sealed, sealing stops the stone from breathing in everything along with protecting it's surface.
I have a rare granite in my kitchen that is 15 years old, It is the hardest stone in the world, I never sealed my granite, now it's a completely different colour. It's gone dark, I like it but thats not the point, had I sealed it way back when, it would would still be the same colour. My granite is also pitted, which is the nature of the beast.