Merlin's key, also known as Merlin's jewel, is an item in Princess Gwenevere and the Jewel Riders. It is special magic jewel that normally is placed atop of Merlin's staff and can be transformed to a key to the Jewel Box that holds the Crown Jewels.
Whenever an Enchanted Jewel is found by the Jewel Riders on their Jewel Quest, Princess Gwenevere touches it with the key and it then returns to the box. Unlike the case of Enchanted Jewels, the staff and its jewel are not the direct source of Merlin's power.
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The following part of the article contains plot spoilers.
Transformed into the key form, and almost stolen from the wizard Merlin by the evil Lady Kale when she banished him into the Wild Magic, it was barely saved from her by a hawk and Sunstar the unicorn in "Jewel Quest, Part I". The key was then given by Merlin to Princess Gwenevere in "Jewel Quest, Part II" to safekeep it from Kale and help Gwen in her mission to secure the seven lost Crown Jewels.
To get the key, Kale kidnaps Sunstar as a hostage in "Jewel Quest, Part II" and later also briefly captures Gwen in "Badlands". Gwenevere uses the key at the end of most Crown Jewel episodes. Kale eventually seizes it from Gwen in "Revenge of the Dark Stone" and then directly from Merlin along with the Jewel Box in during their battle within the dimension of Wild Magic in "Full Circle", where it is again in the staff form. It is then soon destroyed together with Kale in the trap set up for her by Merlin, however leaving the wizard himself stranded inside the Wild Magic.
Merlin's key inspired the key to Avalon in Avalon: Web of Magic.