As you said, just put the diamond on the nail polish before it dries. If it is not firm, it is easy to fall off or go out of shape. AB glue used in jewelry factory can remove diamonds, but it will also leave traces on the surface. If you are worried that sticky diamonds will scratch the surface of the mobile phone, it is best not to stick diamonds on it, because the glue has chemical solvents and the mobile phone case is mostly plastic. If the glue is not firm, it will have some influence on its surface.