The Crystal Jiggy is an artifact found in Jiggywiggy's Temple. It is guarded by Master Jiggywiggy - the leader of a sect of priests devoted to the Crystal Jiggy - and his disciple. The Crystal Jiggy is about four meters in length and is composed of some sort of blue crystalline material. While it appears to be immensely powerful, Banjo and Kazooie only ever see it used to open the doors to various worlds.
In Banjo-Tooie, the Crystal Jiggy is activated when Banjo completes one of Jiggywiggy's challenges, which consist of solving puzzles at the Golden Monolith in Jiggywiggy's Temple; each puzzle depicts an image from a locked world. When activated, the Crystal Jiggy sends out a beam of energy which opens the door for the world which corresponds to the puzzle solved.
In Banjo-Kazooie: Grunty's Revenge, the Crystal Jiggy is flanked by two giant statues of Jiggywiggy. When Banjo and Kazooie have collected a sufficient number of Jiggies, Jiggywiggy gives them a special Jiggy to take to the crystal. This activates the Crystal Jiggy, and it again opens the door to one of the game's worlds.
The Crystal Jiggy, from the time it is seen in Grunty's Revenge, to the time it is seen Banjo-Tooie, seems to grow. Why it grows, or if it actually grows, is unknown.