I can think of three places in Metal Head you are likely to miss gems.

1. From the wooden bridge over the green water look to your right, there's a broken gate. glide to the ledge in front and go through the hole. Behind there are some gems, a key, a life and a whirlwind that takes you to more gems.

2. In the second room where you fight Metal Head to the left are some ledges along the wall that lead to a small room behind the "water" fall.

3. After defeating metal head go inside the building behind him. There is a whirlwind that take you up to the ledge that goes around to a "window". go out the window and walk all the way around the outside of the building.

Hard to glide to Gems - There are some gems that are pretty hard to get to in the room behind Metalhead. Make sure you do a walking jump and jump and glide at the last second. I find it easier to jump from the short side. The side with the whirlwind not the side with the window. I guess Insomniac realized this was too hard since these gems were moved in the Japanese version.

Invisable Banana Boys - You think you are hearing inviable Banana Boys or Strong Arms around the last temple? This somtimes happens if you defeat Metalhead quickly. If you are missing gems and can't find them leave and come back, but take your time.