The Keystone App is a cypher that is used to guide viewers through Guidestones.

The Keystone App is a cypher that is used to guide viewers through the interactive series, Guidestones. Once you find the correct clues that are hidden throughout the series, you can enter them into the keystone. This will get you one step closer to solving the mystery of the Guidestones. The hidden clues are indicated when the "Guidestones ringtone" on Sandy’s phone rings in the show. This means that you are close to a clue. Find the clue, enter it into the empty field in the Keystone App and wait to see what important information is revealed to continue your journey alongside the characters. According to legend, if you complete the keystone it reveals the date of the Apocalypse.