First gold diamond: Given to you when you catch Boo number 50.
Second gold diamond: After you fight the dog outside the mansion, water the seed under the balcony. It will sprout. Return after going to the next area, water it again, and it will grow again. Return again after beating the Giant Boo on the balcony, and water it again. It will spit out a lot of bills, coins, and finally a gold diamond.