The pendant, as you will quickly learn, has eerie powers. You won't find out exactly why for quite a while through your first playthrough, and even now there's some debate as to what exactly the pendant is and where it came from; however, you might find that less important than simply finding out what exactly you can do with it.
The sealed chests and doors can be found in a wide variety of locations throughout the game, though you might not recognize them as such right away in your first playthrough. They are the same size and shape as regular chests, but look quite different, even magical, with their black construction and gold trim. These chests can't be opened right away, even though you will have the pendant from nearly the beginning of the game; you will have to take sidequests much later on in order to loot the treasure. Given that you can't do it right away, you'll imagine that the items in the chests are quite nice—and in fact they are.
The chests and doors can be found are below; note, however, that some elements are upgradeable. We'll explain upgradable chests and doors just after the list.
Red Vest. Upgradable in the Present.
White Vest. Upgradable in the Present.
Black Vest. Upgradable in the Present.
Blue Vest. Upgradable in the Present.
Nova Armor. Upgradable in the Present.
Kali Blade. Upgradable in the Present.
Siren. Upgradable in the Present.
Safe Helm or a
Swallow.
Moon Armor. Upgraded item.
Shiva Edge. Upgraded item.
Valkyrie. Upgraded item.
Red Mail. Upgraded item.
White Mail. Upgraded item.
Black Mail. Upgraded item.
Blue Mail. Upgraded item.
Lumin Robe.Upgrading the chests is an easy process, with no enemies to fight and no real puzzles to solve; however, it does require a fair bit of travel. To upgrade any chest that will change over time, simply follow these steps:
Note that these steps must be followed exactly, especially step two. If you do not inspect and then decline to open, and then open the chest in the Present, you'll still get the original item and you'll not have another chance to upgrade the item without reverting to a prior savegame.
The pendant does plays other roles as well as reaping treasure, most notably in Crono's trial and in the events taking place in the Dark Ages. As we have other guides to give you more information about those roles, please check them out to learn more about the mysterious pendant.