I found a good resource for stone - some textures, lots of info

[voltron7]

While looking for some examples of certain granites, I found this site that maybe useful for some smaller res maps, but it has loads of examples with ruler in pics as well as a lot of information about the different varieties, in general.
The link was from the Marble Institute of America, so I am guessing the reference images are decent representations.

http://www.natural-stone-database.com/

[Neilg]

Seems pretty good. I worked on a job recently with a few super obscure stones (only from one small quarry in france and produces a tiny amount per year kind of thing) and they had good reference of all of them.
Not usable as they are - but good to use as a reference when making a new one from 10 others in photoshop.

[voltron7]

Yes, well put...I have not tried to use them, just helpful...
BR