Olympian Silver is a metal used by Vulcan to craft items. It first appears in Pegasus and the Fight for Olympus.
He began using the metal after giving up on using Olympian Gold, due to it being poisonous to Nirads (who had made peace with the Olympians after being freed from the Gorgons).
After her Olympian Gold leg brace was taken by the Gorgons, Emily was given a new one made from Olympian Silver by Vulcan.