Mythrill

Mythrill is a metal found in remote places on Erathes. It is resembles silver but is stronger and lighter than steel. The name mythrill comes from two words in the Alfar language—myth, meaning "grey", and rill meaning "glitter". The Alfar first made an alloy from it called ithildin ("star moon"), used to decorate gateways, portals and pathways. It was visible only by starlight or moonlight.
MYTHRILL: A rare metal, mythrill is forged like steel but weighs much less. Mythrill reduces the Minimum Strength of weapons, armor, and shields one die type (this stacks with the masterwork bonus) and the weight by half. Weapons and shields made primarily of mythrill cost 100 times normal, while armor costs 10 times normal per piece.
Mythrill weapons count as silver against creatures with a silver Weakness.