The Jewel Box is a special item in Princess Gwenevere and the Jewel Riders. There are actually two of them, one for the Crown Jewels in both seasons and another one for the Wizard Jewels in the second season.
It is where the seven Crown Jewels of great wild magic were placed together in by the wizard Merlin, connected by a magical matrix, until it was stolen and opened by the evil Lady Kale. The box is vital for the Jewel Quest and can be accessed only using Merlin's Key. A new Jewel Box for the Wizard Jewels is introduced at the series' very finale.
In the show Edit
In the first season Edit
Merlin used to be magically bound to the Jewel Box by his magic key. When he gave up it, Merlin broke his ties to the physical world and now cannot return to the world and entered the Crown Jewels' matrix, which was then broken by Lady Kale when she tried to seize the jewels.
Magic all over Avalon will be unstable until all the stones are found by the Jewel Riders in their Jewel Quest and replaced in the Box by Princess Gwenevere. Until the balance is restored, the jewels belong to their lands and are causing wild magic outbreaks.
The more lost Jewels that are found and brought to the Jewel Keep in the Crystal Palace, the more Avalon can resonate with the power of good. The Jewel Riders hope that if all seven stones are found and replaced in the Jewel Box, Merlin might be transported out of the wild magic, fully restored, and brought back home.
In the second season Edit
At the end of the first season, the Crown Jewels return to the Box and their magic saves Avalon and is then tuned to the Jewel Riders. It then gets again stored at the Jewel Keep.
A new Jewel Box is introduced only during the second season's finale, already containing six Wizard Jewels. (The draft script for "Lady of the Lake" described it as a golden case, but in the show it is a square emerald box with a glass dome cover.)
The box is given to Princess Gwenevere by Queen Anya and King Jared to get it to the Heart of Avalon to set Merlin free and defeat the witches. Gwen succeeds to vanquish Kale, but then Morgana steals it away, leading to the events of "The One Jewel".