The New York City Ballet did some diamond mining at the New York State Theater (April 25-June 25), when performances featured the sixth Diamond Project, a sporadically offered series of premieres established in 1992 and named after the philanthropist Irene Diamond. Dancegoers, however, tend to regard the balletic “diamonds” as potential theatrical treasures, and they do create excitement. This season, seven choreographers participated and, as I post this notice, not all have staged their works. But let’s take the choreographers thus far one by one.