As far as yesterday's pic:

My electrochemistry is a little rusty (pun intended), but I have a few tentative ideas.
1) Depending on the rock, it could create a high pH environment. The oxidation of iron requires H+ ions. High pH = low H+ ion concentration and therefore it's a good rust inhibitor.
2) Again, depending on the rock, there may be another mineral with a more positive reduction potential acting as a sacrificial electrode - much like the one in a water heater.

Help.