Shells appear as small groups of sharp rocks with a faint glow emanating from their centre when empty, or with a sealstone balanced on the tips of the rocks when full. They look very much like miniature versions of springs, which is possibly what they are. Unlike daises, which are always featured, shells only appear on dungeon maps when they are full (symbolized by glowing circles). Otherwise, they are invisible.
Location and mechanicsEdit
Shells are introduced in Chapter 3, at the same time as sealstones. There is at least one shell in every dungeon. Additionally, if an enemy is carrying a sealstone, a shell containing the sealstone will appear after the party defeats it. The sealstone itself will only be visible on the minimap once you enter the room that particular enemy is located in and before you defeat it (ie. before a shell is formed).
Shells isolate sealstones from the environment, thus neutralizing their effects while still acting as an appropriate support, without which sealstones cannot exist. Therefore, shells are one of the means for the party to dispose of undesirable sealstones. The other option is restoring the sealstone at a spring, if the party wishes to reuse the sealstone in another dungeon.
Considering their visual resemblance to springs, it is possible that shells function in a similar way, by being connected to the roots of Yggdrasil. Being smaller, they could presumably channel enough power from the World Tree to sustain the sealstone and contain its power, but not enough to physically absorb it. However, this does not explain how or why shells form after an enemy carrying a sealstone is defeated. It is possible that, since the enemy no longer acts as a support for the sealstone, shells spontaneously form to prevent it from disintegrating, but there is no definite evidence of this.