Flame it with a torch.

Thanks Mich...but husband tried this and it was sort of dangerous. This technique causes pieces of the granite to explode and fly off the rock. So we just decided to let nature take it's course and hopefully in time bleach out the iron look. If nothing else, it looks pretty awesome as is....when you get close to the rock you can see some of the black granite.

Silly thing to suggest, i'm sure, but have you tried Lime Away or CLR? Might have to saturate it and try a couple times, but I bet that will help it along faster than weathering alone.

Since your rock is black? Automotive paint stores (that sell to auto body shops) carry an inexpensive chemical rust converter. It's a watery liquid, usually in a spray bottle. It converts rust (iron oxide) to iron phosphate (I think) which then turns black in about 24 hours. It stops - kills - rust too. One brand name is Rust Mort, but they are all the same stuff.

Hi Gardenlady,
Sometimes a rock has rust stains because of something that laid against the rock (such as another rock). However, sometimes the iron is inside the rock and gradually leaches out. In this case, the rock may have shown no rust stains when it was first unearthed, but later may show up as the iron leaches to the surface.
Several products may help remove the stains. A common one available at most hardwares is Iron Out. Mix it into a paste and scrub it onto the rock. Let it set for a while and then come back and hose it off. Another product which is designed for masonry is RustAid. Check at any masonry supplier, they'll have a product.
Two final methods are muriatic acid and sandblasting. Use safety with the acid. Also, the acid will kill your grass. If you sandblast it, it may take the shine off the rock. But it will remove the rust and will brighten up the entire surface of the rock. Don