Just sand the riser down working from 80 grit paper to 320 grit paper to take out any scratches the 80 grit might put in..
When you have you have it back to bare Cocobolo wipe it down with acetone then seal the Cocobolo with super glue spreading it and working it in with your finger ( dont stop the movement with your finger otherwise you will stick to the riser)
Let the super glue dry over nite sand back with 320 grit paper till it's nice and smooth the get a can of Minwax Helmsman Spar Urethane spray on a coat let dry lightly sand back with 320 and apply another coat and it will be as good as new.
The one thing you have to do with Cocoblo and other Rosewoods is seal them otherwise the oil thats in the wood will seep out discoloring it.