The shark's tooth necklace can be obtained after playing the Fishing Trawlerminigame. It can be made by using 5 Shark teeth obtained from the minigame, with some thread. A Crafting level of 91 is required to make it, and doing so will give 170 experience. Crafting the necklace gives a message You string the teeth onto a necklace. It looks snazzy.
Wearing the necklace will give the following effects:
The heal over time effect given when a baron shark is eaten is extended for an additional 5 seconds, giving it the potential to heal 2750 life points instead of the normal 2500.