Photoshop :: Dirty Stone Floor Texture

I'm creating a level for a 3D game, and I'd like to use self-made textures for that one. A part of the level is supposed to take place on the outsides of an old castle. So I need a stone floor that looks old, a little dirty and bumpy.

I've folowed a few tutorials, and I made pretty nice things, but I can't seem to make the stones look more realistic. If you take this example (not created by me)

You'll see that the stones have very subtle cracks and scratches, and that it looks dirty (notice the greenish moss growing on it, darker patches here and there, etc)

I want stones that have similar effects. Can anyone tell me how to make that greenish moss effect and how to make outside stone floors look dirtier and older in general?