Adding the Eternal Gem to the in-game slayer monster drops due to it not currently being one.
Being added to the slayer helmet as an attachment by a scroll purchased with slayer point, or by paying the Slayer Masters a certain amount of GP. Some possible uses for the Gem could include
Adding it to the slayer helmet for an on screen task counter
Unlimited teleports to the slayer masters
Unlimited teleports to popular slayer caves/locations.
Could possibly increase the chance to encounter superior slayer monsters.
Allow a small chance to skip a task without points required, (a small chance due to donator status reducing amount to skip)
The gem could also recolor the helmet serving as a "boss" head dropping by the Superiors
Ability to contact the Slayer Master through the helmet to receive a new task, or skip the current.
Personally I think the Gem should be untradeable. Thanks for reading, and I look forward to everyone's input