There are 4 gems to find in the game, each one hidden in a different floor of the map. If you have all gems with you before reaching the final part of the game, you'll face a secret final boss and unlock the true ending.

There are CDs in the game that give hints about the location of the gems, but in case you need an extra help, this guide has you covered.

(Blast upgrade required)

In the arena, open the door and use a blast shot to destroy the barrier. Use the hook to reach the platform and destroy the wall with a charge shot. Get inside and use another charge shot to reveal a new path. Walk forward and then right to find the gem.

(teleport upgrade required)

In the scientist's lab, use a charge shot to destroy the wall behind the vines to the right of the portrait. Use the teleport to reach the hidden room and collect your prize.

(blast upgrade required)

In the general's quarters, in the wall facing the general's corpse, use a charge shot to destroy the wall between the two missile engravings. Then use a blast shot to open the rest of the way and get the gem.

From the north elevator, go to the east until you reach a room with many chairs and corpses. Use a charge shot to destroy a secret wall marked with two red vertical pillars. You'll find the gem inside the hidden room.