An Irishman by the name of O'Mally proposed to his girl on St. Patrick's Day. He gave her a ring with a synthetic diamond. The excited young lass showed it to her father, a jeweler. He took one look at it and saw it wasn't real.
The young lass, on learning it wasn't real, returned to her future husband and protested vehemently about his cheapness.
"It was in honor of St. Patrick's Day," he smiled. "I gave you a sham rock."