A rare fossilised tusk is a fossil that can be obtained by cleaning an unidentified rare fossil on the fossil cleaning bench east of the Museum Camp on Fossil Island.
They can be used at the mycelium pool to be calcified into a rare enriched bone via a chemical process that uses pyrophosphite and calcite. Whilst the reaction is happening in the pool, players must add additional pyrophosphite and calcite to maintain the chemical process, and pop bobbing fossils that surface around the pool, otherwise the fossils will become too exposed to air and dissolve. Once completed, players can open the sluice gate behind the Ancient Shroom, and check the rinsing pool to obtain the enriched bones. They are then used on the easternmost strange machine in the House on the Hill for 2,500 Prayer experience.
Also, they can be traded to the Fossil Collector in exchange for 500 numulites.