There are various chests in 600 A.D. and 1000 A.D. bearing the Zeal crest, they're solid black and you get a message saying they're sealed by a mysterious force if you try to open them without a charged pendant. In addition, there are also several sealed doors in 2300 A.D. which can only be opened by a charged pendant.
You will have a charged pendant from Break the Seal! onward, and you'll have the ability to fly from The Time Egg onward. Some chests can be reached with just a charged pendant, but some require you to be able to fly as well, so for the sake of convenience they have been separated into groups.
One final general rule for sealed chests is that the ones which occur both in 600 A.D. and 1000 A.D. can be received once, or twice with an upgraded form. If you open a chest in 600 A.D., you'll get the item but it won't appear in 1000 A.D. If you open a chest in 1000 A.D., you'll get its item and you can open it in 600 A.D. as well. However, if you try to open a chest in 600 A.D. and select "no" when asked if you want to open it, then try to open it in 1000 A.D. the item inside the chest will upgrade into a more powerful form. You can also then go back and get the non-upgraded item in 600 A.D. In general therefore, you should access every chest you might want to open in 600 A.D. first, get its upgraded form in 1000 A.D., then go back to get its non-upgraded form in 600 A.D. Also please note that some chests only show up in one time period, and hence have no upgradeable form.
How absorption works and armor comparisons
Each elemental armor and their upgraded versions absorb damage for that particular element. That is to say, the character wearing it will receive no damage, but will instead be healed for a portion of the amount. That portion is determined by whether it is a vest or a mail. Vests absorb 50% of the damage (that is, heals the wearer for 50% of what would normally be taken as damage), and mails absorb 100%.
The one exception to this rule is the Ruby Vest, which blocks 50% of fire damage. This means that the incoming fire damage is reduced by 50%, or halved. No absorption occurs.
Chests accessible with only a charged pendant
- Go into Guardia forest and examine the upper right portion of it next to the sign that says, "Dead end." The non-upgradeable sealed chest there will have a Speed Tab.
- Head into the Magic Cave that led to Magus's lair; close to the entrance is a non-upgradeable chest. Inside is the Magic Ring, an accessory that gives +6 to Magic.
- There's a non-upgradeable pair of items found together within a single chest in the Heckran Cave. The easiest way to get them is to start by going to the Vortex Pt near Taban's house and head down through the waterfall until you reach the sealed chest. Keep going until you see the whirlpool, and jump in to end up back where you started. You'll get a Wall Ring and a Dash Ring for your troubles.
- There's a single sealed chest with a Power Ring in Guardia Forest. The problem is at this point in the game the path is sealed up, so you can only reach it by going through the time portal which leads there. If you went through the portal earlier and connected it to the end of time, great, otherwise head through 2300 A.D. until you reach the Bangor Dome and you'll open the warp to Guardia forest.
- Head to Medina Village and go to the northern diamond-shaped object (the northern palace) to meet a Nu from 13,000 years ago; you'll be offered a choice between a powerful sword and a powerful defensive armor. The box to the right is the Swallow, a sword for Crono that is better than any weapon you'll find until you get the Epoch in The Time Egg, but the helmet is the Safe Helm, which cuts physical damage by 1/3, and only offers less defense than the Prism Helm and the legendary Ozzie Pants. So basically, I'd recommend the Safe Helm in the left chest. Note, however, that if you get the Swallow here, you can still charm a Safe Helm later from the Lavos Spawn in the Black Omen (but not the earlier ones in 2300 AD), but the Swallow cannot be obtained any other way.
Sealed Doors in 2300 A.D. (the Future)
In 2300 A.D, there are several sets of sealed doors which offer rare items, generally one in each dome.
- Take the transport to Bangor Dome from the end of time, which you should have opened already, and open the door in the back. In addition to a Full Ether healing item, you'll get the Charm Top, an accessory which improves Ayla's chances at using the charm ability, and you'll get the Wallet, an accessory which allows you to turn experience points into cash.
- Directly south of the Bangor Dome is the Trann dome. Open the door in the back, and you'll get the Gold Stud which cuts all magic use by 75%, a Full Ether, and a Magic Tab which you can pick up off the ground.
- In the Arris Dome, where you fought the Guardian and found the seed, there's a sealed door in the area beyond the door on the right (the sealed one you opened by interacting with the console and pressing ++). The door is in the very top right corner. Inside you'll find a Lumin Robe which provides excellent defense for the girls, a Hit Ring which raises your strike by 10, and a Gold Erng (earring) which raises your Max HP by 50%, an Elixer, and you'll also find a Power Tab on the ground.
Note: Upgradable Chests actually provide you with two items. It is a three step process. Select the 600 A.D. Chest but decline to take it, then make sure you grab the upgraded 1000 A.D. Item and then return for the 600 A.D. Item.
- In the Elder's house in the village of Porre in 600 A.D. there are two sealed chests. Make sure to select them but decline to take them. Head to the Mayor's house in the present to get the upgraded White Mail and Black Mail. Go back in time again to pick up the White Vest and Black Vest. White Mail and Black Mail absorb 100% of Lightning and Shadow damage respectively, whereas White Vests and Black Vests only absorb 50% of those respective elements and offer worse defense.
- Go into the Truce Inn in 600 A.D. and select the chest there but decline to take it. Go to the Truce Inn in 1000 A.D. and open it to get the Blue Mail. Head back to 600 A.D. to pick up the Blue Vest. The Blue Mail absorbs 100% of water attacks, whereas the Blue Vest only absorbs 50% of water attacks and has worse defense.
- In 600 A.D. go to Guardia Castle, through the throne room and up the stairs on the left side. Select the chest and decline to open it. Go to 1000 A.D. and open it to get the Red Mail. Head back to 600 A.D. to get the Red Vest. The Red Mail absorbs 100% of fire attacks, and the Red Vest is weaker and only absorbs 50% of fire attacks. Note that the Red Vest is often confused with the Ruby Vest, which blocks 50% of fire damage, but the character takes the other 50% of the damage.
Chests accessible with the Epoch
There are also three sealed / upgradable chests in the Northern ruins which you can reach after completing Frog's Sidequest. As before, decline to open them in the past (600 A.D.) and open them in the present to get a powered up version, and open them once again in the past to get the low power version.
- The Nova Armor will power up to become the Moon Armor, which raises magic defense by 10. The Nova Armor protects status.
- The Siren will power up to become the Valkyrie, a better weapon for Marle. The Siren also casts "Stop" randomly.
- The Kali Blade will power up to become the Shiva edge, a better weapon for Crono which does four times damage on critical strikes.