Sounds counterintuitive, but have you tried taking photos in the dark using a flash? You may need to play with the angle and distance you are taking the photo from, to get the right level of reflection/minimise whiteout, but you may be able to pick out the scratched part from the background this way?
Alternatively, the image-processing software on the iPhone seems very good at picking up blemishes against a constant background - again, subject to getting the right angle/reflection.
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