If you have a jeweler's optic, take a good look over it to see if you notice something interesting. If you have a pinpointer, it might direct you to one part of the rock over another. If you have have muriatic acid (HCL), you might want to give it a bath in diluted HCL (perhaps 4:1 water:HCL) to see what happens, as it will remove any calcite and should clean up the iron stain but won't affect quartz, silver or gold. Or instead, Super Iron Out will just remove the iron stain. As stated above, breaking the rock and looking at the fresh break is not a bad idea either. Good luck!