Hey this is a huge problem I found all 50 boos and there are 2 gold diamonds..

You do need to collect all 50 Boos in order to get the second gold diamond. I seriously doubt there is something wrong with your game. A more reasonable solution could be that when the diamond appeared it fell behind something and/or you never picked it up. However, if your game is messed up, you could attempt to return it and get another copy (unless of course you bought the game when it came out) and try playing that with the same memory card file. Or something could be wrong with the memory card file. It could possibly have an error in which the game thinks you've collected the diamond, but never adds it to your totals.

You need 100,000,000G for the A rank mansion.
(A hint on finding gold: I have found that in Area 4 or another area a long period after you clear the rooms, all the gold comes back, so you can reopen all the cabinets and stuff to get it all again. This is an easy way to get the A rank mansion.)

You should have received the second Gold Diamond when you captured Boo #50.
P.S. To get an A Rank, you need to have 100,000,000G when you complete the game!

Are you sure you got the gold diamond from collecting all the boos does your thing say 50/50 because king boo doesn't count.

You need at least 100,000,000 I think for an A Rank

If you save after collecting the last Boo without taking the diamond, it will be gone for good...or at least until you erase and replay your gamefile.

Make sure you save after getting each Boo. But when you capture the last one, don't save the game. Then you can re-catch it if you mess up.

This has happened to me before. I don't know whats wrong but it is annoying. When you catch that boo you might be in the artists room. Probably. You could have missed a boo which makes it extemely hard to find if you don't catch a boo whenever you see one. I think the problem is, the game doesn't like you. :P

I BELIEVE YOU HAVE TO GET THE LAST BOO WITHOUT LOSING HEALTH