Most people know that 28.35 grams is 1 Ounce, but that is the avoirdupois Ounce, the common method to weigh groceries etc. When you are weighing precious metals or stones, jewellers use another method called Troy in which 31.103 grams is 1 Troy Ounce. So remember:
28.35 GRAMS = 1 OUNCE (WHEN WEIGHING GROCERIES)
31.103 GRAMS = 1 OUNCE (WHEN WEIGHING GOLD)