From the Super Mario Wiki, the Mario encyclopedia
The Triangle Jewel is a special item in Paper Mario: The Origami King. It is located in Scorching Sandpaper West, buried between two specific cacti. Mario must locate the two cacti near the middle, close together but leaning away from each other. He must have Professor Toad dig between them to unearth the jewel, which he can then use to activate the nearby tower. The inscription at the top of the tower refers to the jewel as an "eye" and the cacti as "twins entwined in needles".