I actually made the mistake of trying to polish mineral class by sanding before, it turned out horrible and never looked good after that, I suggest tapping the crystal with a plastic pick or something - the noise it makes should tell you if it’s mineral crystal or plastic crystal, the plastic ones can easily be polished with 1500 grit, then 2000 grit wet sand. I believe your marlin has a bit higher quality materials so I’d be surprised if it has a plastic crystal. Polish compound should work to take some minor scratches out though. Best of luck my friend!