@ Dave

I tried this way before, but doesn't looks real . the grain are too small and all of the them the same size.

But in Maya mental ray shader there is Car_paint_X which include some of the flake.
I'm trying to find out

@ New design
I do have camera, but I live is desert, No river. Hardly to find a rock. I can buy rocks from shop, but they don't have this one.

So I think my option are photoshop or Car_paint (Mental-ray material)

if you know something , say something,

regards