Shard of Eden
The Shard of Eden was an artifact recovered by Connor Kenway during the American Revolution. Similar to a ring in design, the Shard was a Piece of Eden that projected an electromagnetic field that acted as a kind of shield, able to protect its wearer from musket fire.
The Shard was formerly in the possesion of the renowned privateer William Kidd, and its capabilities contributed to stories of Kidd's invincibility. Kidd hid the Shard on Oak Island, to keep it safe from Templar forces seeking it out, Connor was able to pinpoint the location of the Shard's resting place by piecing together a map Kidd left behind.
The shard is stated to repel metal objects, whether this only applies to ferrous (magnetic) metals or all metallic solids is unclear, however the shard does not appear to affect bladed weapons such as swords, nor does it affect environmental objects (except in cutscenes, it is shown repelling a hipflask out of the holder's hand with ease)