do these affect pH??

granite won't in my experience. its usually mostly quartz, which is really hard and largely inert for aquarium purposes...

marble statues in europe are hundreds of years old, so i'd say negative on that too.

Anything your choosing to place in the aquarium can be tested by using muratic acid. Put a little muratic acid on whatever it is your looking to place in the aquarium, if it doesn't fizz it's inert and safe to use. As for your question I'd say that granite is safe, but I would test marble, since marble's basically limestone and softer than granite. Yes limestone will definatly alter to water parameters.

I would double check on the marble. Marble is limestone (or sandstone...I forget which. Pretty sure it's limestone) that has been super-heated under pressure into its final form. It definitely reacts with chemicals. One of the major problems facing those old marble statues and buildings in Europe is that they are dissolving away due to increased pollution and acid rain. Again, I don't know for sure but you might want to double check on the marble.

Granite, for the most part, should be fine. There might be some rare forms of it that have pyrite in it........you might see this as little sparkles in the stone (not really sure. I'm not a geologist. I know it occurs in other stones, though). That's bad news and you don't want it in your tank. Like I said, that's probably pretty rare.

[edit]And Tre beat me to it by just a hair. Now you have twice the advice.