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To add a point (as a woodworker and a shaver), I have used a bunch of different green compounds for buffing and woodworking, and they all seem to have aluminum oxide or another "sharp" abrasive in them to make them cut a little faster.
For razors, the green pigment cuts slower and makes a finer edge. It's something like $12 on ebay for 3 1/2 ounces (star shaving, and maybe anyone else who sells it), which should last a long time. Just be careful not to get it on things other than the strop, because whatever it gets on will be green and if you try to rub it off with a rag, you'll just end up rubbing it in.
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