Fourth clue

Exploration Stage: Amusement Park
From: Rayden Storke
Description: Let's hurry. The city is on the verge of collapse. We are running out of time before it is irreversible. Will the emerald reveal the whereabouts of the next key piece? Hmmm... maybe it's the emerald glasses. Keep looking.
Completed: You found the emerald glasses. But how do you figure out the secret with these? That's right, try applying ink developer to the lenses... it works! Look at this message: "Only a pure heart with long ears can guide one."