The rod clay mould is a quest item made by the player during In Aid of the Myreque to craft the silvthril rod, which later becomes the Rod of ivandis. It is made by using a piece of soft clay on Ivandis' tomb under the Hair of the Dog pub in Canifis (the first cave entrance after walking through the secret door).
If it is lost, another may be obtained from a crate in the Myreque Hideout in Meiyerditch, in the room east of the entrance ladder. Drakan's medallion provides a convenient teleport to the hideout.
It can be added to the tool belt as of 11 April 2016, as part of the release of the Morytania updates.