how can get a better colour of diamond from rough to polish?

A better color would be a whiter (lighter) color. For example, from an H- color to an F- color. This is not possible. You can influence the color, by heat and at the same time a specific dose of radioactive beams.
This is done in order to produce “artificial fancy colors” (Blue, Green...etc.) A combination of certain temperatures and certain radioactive beams (Alfa, Beta or Gamma Rays) will produce a certain color. This is done on a reactor and one can easily determine afterwards, if a colored diamond is natural colored or men-made, by holding a Geiger- counter against the stone.

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