What does the diamond shaped stamp on a ring mean?

The letter, whether it is upper case or lower case, and the shape surrounding it, combine to indicate the year a piece of gold was formed. The idea came from the practice of having a different assay master appointed each year. The assay master held office from May to April. To match a precious metal piece to the assay master who oversaw its assessment, a system of revolving letters was used, one per assay master's one-year term. Twenty letters of the alphabet were used; J, V, W, X, Y, and Z were not included. Each letter was used in turn, five times per century. Each time the letter came into rotation it was varied in style or in the presentation of its surrounding shield.