The Enchanted gem is an item used in the Slayer skill. They are sold by all Slayer Masters for one coin. These allow a player to contact the slayer master that has assigned them their current slayer assignment, where they can ask for tips, ask who the master is and where they are located, and most importantly see how many monsters they still have left to kill for their slayer assignment. An alternative to the gem is a worn Combat bracelet, which will update the player every 10 kills if it has 1 or more charges. You cannot ask for a new Slayer task through the Gems, you have to go talk to the Slayer Master in person. The Enchanted gem has a right-click option to quickly check how many monsters are left for your assignment. Before that update, checking how many kills left would be interrupted by combat, so you had to wait until the monster was defeated before checking it.
Since the Social Slayer was introduced on 11 July 2012, players can now invite each other to simply speed up the slayer assignment by using the Ring of Slaying or an enchanted gem targeting on the partner. Note that both must be on the same assignment or one of both must have no assignment at all.
The Enchanted gem is referred to as a 'Slayer gem' when all the charges on a Ring of slaying are used. "The ring collapses into a Slayer gem, which you stow in your pack." It is also referred to as such in the message when the player learns to craft rings of slaying.
After While Guthix Sleeps has been completed it is still possible to contact Duradel by asking the master "Have you got any tips for me?" after a task has been completed, if you have the correct requirements they will pass you over to Duradel.
Before the HD update, when contacting a slayer master via the gem, it would show the slayer master in a light blue oval instead of just a face.
If you try to invite yourself to a Social Slayer task, you will get the message: If you want something done properly, you need to do it yourself...but not on this case.