I collect rocks. Because I'm a huge geek.
And that granite sounds really pretty - and being granite is very likely safe as well.
I just had to say, as a general thing - be careful to accurately ID the mineral content of any rocks you find and might want to put in your tank. Some contain minerals that could poison your fish - like the copper in malachite, for example. Calcium-bearing rocks can seriously mess with water chemistry. Other minerals can slowly dissolve in tank water, releasing arsenic, lead or other deadly elements into your tank, especially if your PH is low.
All rocks with 'glitter' or seams of metal in them should be avoided. Many stones sold at new age type shops as decorative or 'healing crystals' aren't great for fish - amethyst has manganese in it, pyrite is quite toxic, etc. Clear quartz is pretty much always going to be okay.
Here's an article on toxic minerals:
And a forum post written by a geologist, which is pretty awesome:
And here's a really comprehensive list of the toxins found in many minerals - the site linked is concerned with the making of gem essences for human & animal consumption, but much of it would apply to aquariums as well.