Height-altering crystals

 can be found deep within the Gravity Falls Forest and, as their name implies, they can change an object's or living being's (Animals and Humans) size when light shines through them. Dipper Pines uses one to create his crystal flashlight.

History
After Mabel Pines teases Dipper for being a millimeter shorter than her, Dipper looks for a way to increase his height. Upon reading about them in 3, he searches for them in the forest, and after tumbling down a hill, finds them. He removes a crystal and takes it home, where he assembles his crystal flashlight and uses it to change the size of a chess pawn and later himself. When Mabel discovers what her brother's doing, she tries to steal it from him and fights for the light. When Mabel accidentally reveals its function to 'Lil Gideon, he uses it to shrink the twins and later accidentally Soos, and later attempts to shrink Grunkle Stan, but to no avail. Dipper and Mabel grow themselves back to their full heights and smash the crystal, but realizing they forgot to re-grow Soos, need to glue it back together.

Games
The Twin Mystery Vortex of Doom (the page in the journal)