The main island of Japan has a forest that has a secret dungeon. Run through the forest until you reach the cave. The Red Diamonds are as follows.

3 Black Guncannons
2 Black Guncannons and 1 Black Gundam
3 Black Full Armor Gundams
3 Black Rick Dias
2 Black Rick Dias and 1 Black Hyaku Shiki
3 Black Gundam MK-2
1 Black Zeta Gundam
1 Black ZZ Gundam

At the end of this dungeon, you will get some very powerful equipment such as the Nu Gundam Rifle and Nu Gundam Sabre.