Question

The gray diamond is not used in jewellery
why?​

Answer 1

This is because that by mathematical definition the round diamond shape causes light go through the diamond (and bounce off it) in an optimal way that causes it to appear whiter - it simply does not sustain the color. Therefore most colored diamonds are cut into radiants and cushions

Most of the gray colored diamonds get their color due to high concentration of hydrogen and in rare occasion's boron like blue diamonds.