Lodestone

A lodestone is a waste product of krakens. Water Ashari implant them in the place of the Aerie of Torrent, a rift between Elemental Plane of Water and the Material Plane, to keep the rift size contained, preventing the merging of the planes, and deter the settlement of Vesrah from sinking.

Description
Lodestones are large, gray, and roughly spherical with some variations. A single smaller dark green stone is embedded within each.

Lodestones are magnetic and are "profoundly magical". Lodestones are naturally connected to magic, in a way comparable to the newer whitestone. They are implanted by the Water Ashari of Vesrah into a location called the Aerie of Torrent, where they keep Vesrah from sinking. The lodestones' power wanes over time. They are also used in tandem with the Ashari's magical skill to control the merging of the Material Plane of Exandria and the Elemental Plane of Water at rifts between the two planes. Lodestones can also prevent krakens from crossing these rifts.

History
Vesrah relies heavily on the lodestones, and while keeping them from sinking is the primary use, the general magic they provide is important to the Water Ashari. The test of the Water Ashari during the Aramenté involves retrieving lodestones from krakens in the Elemental Plane of Water without killing the kraken.

Vilya of the Air Ashari attempted this around 806 or 807 PD as her final Aramenté test, but she vanished and her companions were all killed. During Campaign 1 in 811 PD, her daughter Keyleth also faced this as her final test. She and Vox Machina successfully retrieved three lodestones from Ulugrah.

Other lodestones
Caleb Widogast at times references a lodestone, but he is likely referring to real-world lodestone, ie, naturally occurring magnetite. This lodestone is a component of the spell Disintegrate, Reverse Gravity, as well as Mending using two lodestones.