The black tourmaline core is an item dropped by the Grotesque Guardians that can be found on the Slayer Tower's rooftop. They can be used to upgrade the Bandos boots to create guardian boots. When a player selects the Inspect option on the core, a message in the chatbox will state Fallen from the centre of a Grotesque Guardian. This could be attached to a pair of Bandos boots...
When a player creates the guardian boots, a message in the chatbox will state The second you hold your tourmaline core by the Bandos boots, the core is absorbed rapidly. Perhaps there is some link between the metal in Bandos and tourmaline...
Monster Combat level Quantity Rarity Grotesque Guardians 228; 328 1 Very rare (1/1,000)