A Minion's Scourgestone is the insignia of the Scourge rank and file. These are sometimes found on the weaker of their troops positioned in the Plaguelands, though the term "weaker" is certainly to be considered in context. The Scourge, no matter where they are, should never be underestimated.
The Argent Dawn currently offers a valor token in exchange for twenty of these insignia - an exchange I'm happy to do, provided you have earned enough scourgestones!
Congratulations, <name> - I am pleased to award you with an Argent Dawn valor token!
The tokens serve two purposes. One - they can be used to further your standing within the Argent Dawn; to do so may avail you options and rewards that are reserved for our honored and revered allies. Two - they can be given to our quartermasters, along with coin, as payment for some of the Dawn's more prized items.
Whatever you decide, know that you are earning your commission. Well done!