Found in the deepest mines of Nidavellir, Ysgardian heartwire is a flexible metal that is easily formed into thin strands. Though too soft to make large objects out of, it can easily be woven into chain to reinforce weak areas.
When added to a chain shirt, a suit of chainmail, or a suit of heavy armor, this metal has the effect of providing a +2 bonus to AC against rolls to confirmation checks for critical hits. Ysgardian heartwire is so light that weaving it into armor in this fashion does not increase its weight. Ysgardian heartwire provides a +1500 gp price modifier to items including it in their creation.
- Book of Exalted Deeds, pg.38