Hi Rob,You are correct about the fact the outer surface may not streak correctly. I should have been more specific when I mentioned one should test using a piece from within the rock. On the pieces I have had tested, I was requested to make sure the sample contained a fresh surface at least a 1/4" from the outer surface for analysis.
Care should be taken when cutting the suspected rock also. Cutting oils may contaminate the new surface.
That is why if a person really feels they have a meteorite, they should have it checked by an expert before doing anything.
Reg