Might of been used till they found the rock they wanted and then tossed this one. But I have to ask is that a divot or a chip on the 1 side? Could be a rock for making points if its a divot. I find quartzite cobbles like yours with divots on them. The orange to brown is the patina that is left on the stone after it was used. I find that patina on all of my stone grinders, hammers and anvils. Think it was used but not for long.