First, adopt a Chihuahua. It must be a boy (It works better with blue eyes, though I'm not sure if you can get them for boys.) Adopt it and call it Polari (You can even change the owner's name to Rosalina...) Go to the toy closet and get out the paint bucket. Change the colour to Black. Paint the chihuahua all over.
Also, Polari is very wise, so when he's an adult, train him lots!! Have fun with your new pet!
Diamondz are extremely rare, and I will tell you how to get them. You know the South Sea Island, right? And those little Gold Doubloon things that at first, are invisible? Well, fill your inventory with them, and after a while, one doubloon will turn into a Diamond.
It works with other things like skulls, coconuts and feathers but I've forgotten which one matches up with which jewel. Have fun discovering rare items!