A list of the locations of all sealed chests and sealed doors in Chrono Trigger, including its contents.
Zeal Palace: The only sealed door in this era (no chests). Your party sees Schala enter the door using the pendant, prompting you to do the same. Opening and entering this door are required to advance the story.
In the dead end area as traversed in 1000 AD after the trial.Guardia Castle: Red Vest (can be upgraded to Red Plate)
Located on the way up to the king’s quarters in one of the rooms along the stairs.Truce Inn: Blue vest (can be upgraded to Blue Plate)
Porre Mayor’s Manor:
-White Vest (can be upgrade to White Plate)
-Black Vest (can be upgrade to Black Plate)
Magic Cave: Magic Ring
Near the beginning of this overall short cave.
Hero’s Grave/Northern Ruins:
-Nova Armor (can be upgraded to Moonbeam Armor)
-Siren’s Kiss (weapon for Marle; can be upgraded to Valkyrie Bow)
-Onimaru (weapon for Chrono; can be upgraded to Suzaku)
**To upgrade a chest, interact with that particular sealed chest in 600 AD. The screen will flash blue, and state it’s reacting to the pendant. Then you will be asked if you want to open the box. Choose “No,” then return to the same location in 1000 AD to acquire the upgraded contents for that chest.**
In the dead end area next to the warp gate. Only accessible via the End of Time.Guardia Castle: Red Vest (upgraded to Red Plate if visited in 600 AD)
Located on the way up to the king’s quarters in one of the rooms along the stairs.Truce Inn: Blue vest (upgraded to Blue Plate if visited in 600 AD)
Heckran Cave: Barrier Ring, Speed Ring
Located near the end of the cave. May be easier to reach by entering the cave near Lucca’s house.
Porre Mayor’s Manor:
-White Vest (upgraded to White Plate if visited in 600 AD)
-Black Vest (upgraded to Black Plate if visited in 600 AD)
Hero’s Grave/Northern Ruins:
-Nova Armor (upgrade to Moonbeam Armor if visited in 600 AD)
-Siren’s Kiss (weapon for Marle; upgraded to Valkyrie Bow if visited in 600 AD)
-Onimaru (weapon for Chrono; upgraded to Suzaku if visited in 600 AD)
Forest Ruins: Guardian Helm OR Swallow (weapon for Chrono)
This is the blue pyramid above Medina village. A Nu appears when the pendant unlocks the blue barrier giving you the option to choose only one of two sealed chests.
**To upgrade a chest, interact with that particular sealed chest first in 600 AD. The screen will flash blue, and state it’s reacting to the pendant. Then you will be asked if you want to open the box. Choose “No,” then return to the same location in 1000 AD to acquire the upgraded contents for that chest.**
Afterwards, you can go back to the original sealed chests in 600 AD to acquire the original weapon/armor contents 
This era contains sealed doors instead of chests.
Bangor Dome: Alluring Top, Full Ether, Workman’s Wallet
This is the first place your party ends up after fleeing from the trial in 1000 AD.
Trann Dome: Gold Stud, Full Ether, Magic Capsule
Directly below Bangor Dome. The sealed door is behind the human survivors.
Arris Dome: Luminous Robe, Elixir, Gold Earring, Acuity Ring, Strength Capsule
The dome where the party meets Doan and retrieves seeds. The sealed door is near another door where the party first learns about the Day of Lavos. Accessing the sealed door requires the password acquired from the rat.
Keeper Dome: Wings of Time (required for story progression)
On the other side of the sewer access and near Death Peak. This is where the party can meet Belthasar. You must come here much later to advance the plot.