Q. How is it that a wedding ring must be plain, without diamonds or other precious stones?
A. Some people, even when they are getting married, are not entirely honest with each other. One of the problems of lack of honesty must have been, at a stage in our history, that a groom would deceive his bride as to the value of the wedding ring, and this would affect the validity of the marriage ceremony. Hence to place the value of the ring beyond all doubt, it was ordained that only a plain ring could be used.
Another approach to the answer is to say that just as an ostentatious ring does not guarantee a happy marriage, so in anything in marriage it is not the outward show that matters but the inner spirit of love, harmony and understanding.