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 uses one to create his crystal flashlight.

History
At some point before the events of the series, the author of 3 comes across and studies the crystals, recording their powers, appearance and location in said book.

In "Little Dipper," after Mabel teases Dipper for being a millimeter shorter than her, Dipper looks for a way to increase his height. Upon reading about them in Journal 3, he searches for them in the forest, and after tumbling down a hill, he 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 is doing, she tries to steal it from him and they fight over the flashlight. When Mabel accidentally reveals its function to Gideon Gleeful, he uses it to shrink the twins and accidentally Soos. Later, he attempts to shrink Grunkle Stan, but to no avail. Dipper and Mabel grow themselves back to their full heights and smash the crystal, but after realizing, they forgot to regrow Soos they glue it back together.

In "Gideon Rises," the height-altering crystals page from 3 makes an appearance while Dipper and Mabel are thinking of a way to defeat Gideon and it is later seen again in the end credits.