Credit to PowerPyx on YouTube.
Map: N.Sanity Island / Level: N.Sanity Peak
In N.Sanity Peak when you reach a room with 9 disappearing platforms (they leave behind a blue silhouette) you need to jump on them in a particular order. The first to jump on is the left-most platform. You should hear an audio cue when done correctly. Then jump around the platforms counter-clockwise. Each will give an audio cue, that's how you know you're doing it right. The last platform to jump on will be the middle one. Then the red gem spawns in front of you.
You have to finish the level afterward to unlock the Ruby Red trophy / achievement.