A quick suggestion

[hecklerSL81]

if i have a half diamond that doesnt fit well or work effitiantly .. should i sand it... grind it ... or what else

[Krypos]

make a whole new one.

then you have two.

[cjames73]

if its a standard sized diamond you could use a file to take the front off, not as much as in this pic but you get the idea.

you could also try a metal polishing compound and a leather strop, gives your picks a mirror finish.
or go with Krypos and make your own. theres plenty of templates on here if you search

[xnoobandrew]

well, if its good material or u gave a certain dedication to it, then i'd file it. otherwise just make a whole new other one

[pauly003]

File down the upper tip of the diamond to make it a little smoother to work with.