Rings of Eden

The Shards of Eden were ring-like Pieces of Eden, which were invented by the First Civilization to serve as personal shields for their wearers. The Shards projected an electromagnetic field which protected their wearers from projectile damage, including those fired from energy-based and conventional weapons. Shards could also be used in conjuction with some sort of shield-projection gun, which would create a spherical, blue shield around an area when utilized.

History
A few decades before the First Catastrophe, the First Civilisation attempted to adapt the principles behind the Shards to create shields on the scale of the whole planet, in an attempt to repel the impending solar flare. However, they didn't have the time or resources to make even a large enough shield to safeguard a single city, and so they abandoned the research for other favorable solutions.

At some point, a Shard came into the possession of the renowned privateer William Kidd, and its ability to deflect metal or, more specifically, lead musket balls, contributed to folklore stories of his invincibility. Kidd eventually chose to hide his Shard on Oak Island, to keep it safe from Templar forces who sought it. To this end, Kidd created a map that provided clues to the Shard's location, and divided it among four of his former crew members.

Years later, the Assassin Connor managed to gather the four pieces of the map and, along with Robert Faulkner, traveled to Oak Island to obtain the Shard. Though both wondered why Kidd protected this "treasure" so carefully, Connor soon realized its power when it threw Faulkner's whiskey flask out of his hand.

Trivia

 * Subsequent to Connor's acquisition of the Shard, its effects were present while reliving the memories of Haytham Kenway.
 * The Shard's effect could also be observed on Desmond Miles after he had relived the memory in which Connor found it.
 * The Shard shares many features with the rings of Neodymium magnets that are found in miniature motors, which give off a strong magnetic field.
 * Allthough the Shard deflects meatle, it is unknown why it only deflects musket balls, and not mele weapons.

Reference

 * Assassin's Creed III