Look on the surface, if these are re-deposits try to etch using wet chemicals; in a case of pitches in diamond, I do not see a chance to improve the surface.

MG.

Thank you very much for your reply. yes it seem to me re-deposited . but what kind of chemical etchent is  ok for it?. i have tried hydrofluoric acid and BOE but it is not working.

If deposits include organic parts, try to use sulphuric acid based mixtures.

ok thank you

What gas do you use to etch the crystal? oxygen related? In fact, it's difficult and seldom to wet clean the C-based impurities, and often post dry clean are needed. 

yes i used oxygen, so what kind of post dry clean is perfect for it?