If the acetone won't touch it and it isn't natural mineral streaking in the wood then you have iron based stains. These can be quite easily removed by treating the wood with oxalic acid. Oxalic acid is commonly known as "wood bleach" and can be found at better hardware or woodworking stores. Just make sure and neutralize it well before you continue with your work.

Hope this helps,

Dan

Dan May
www.classicgunstocks.com
www.millersingletrigger.com