Chest of Harmony

The mysterious chest is a chest first seen in Princess Twilight Sparkle - Part 2 after emerging from the Tree of Harmony. The chest requires six keys to open.

Princess Twilight Sparkle - Part 2
When the Tree of Harmony regains its magic, a flower sprouts at its roots. The flower blooms to reveal a chest with six locks. Princess Celestia discloses that she does not know where the keys are but assures Twilight Sparkle "...it is a mystery you will not be solving on your own".

Castle Mane-ia
The chest is briefly mentioned in the beginning of the episode. According to Twilight, no information on the chest and its contents exists in Ponyville.

Twilight Time
A blueprint of the chest hangs on a wall in the Golden Oak Library.

Maud Pie
The same blueprints as in Twilight Time are seen again in Maud Pie.

Keys
Following the time of the chest's discovery, Twilight and her friends come into possession of several objects of assorted value. The ponies acquire these objects through a lesson they teach others or learn themselves that corresponds to their respective Element of Harmony. Prior to the acquisition of these items, each pony has a moment of clarity relating to their element that manifests as a rainbow-colored glow in their eyes, and the items themselves emit the same glow soon after the ponies receive them.

Rarity – generosity
In Rarity Takes Manehattan, Rarity wins Manehattan's Fashion Week, despite the schemes of fellow designer Suri Polomare and her own treatment of her friends as a result. Designer Coco Pommel is so inspired by the generosity shown between Rarity and her friends that she quits her job as Suri's assistant and seeks out a better life in the big city. As a sign of her gratitude, Coco gives Rarity a gift: a spool of rainbow-colored thread.

Rainbow Dash – loyalty
In Rainbow Falls, Rainbow Dash confronts Spitfire and Fleetfoot of the Wonderbolts over their unethical means of recruiting her to Cloudsdale's aerial relay team and chooses Ponyville over them. Spitfire and Fleetfoot, inspired by Dash's loyalty, apologize to their teammate Soarin for trying to replace him. As a token of her appreciation, Spitfire gives Rainbow Dash a golden Wonderbolts pin.

Pinkie Pie – laughter
In Pinkie Pride, Pinkie Pie feels challenged by fellow party planner Cheese Sandwich and allows her pride to get in the way of planning Rainbow Dash's birthday party. When Cheese Sandwich makes it clear that he never wanted to take Pinkie's place in Ponyville and the two make amends, he gives her his rubber chicken companion Boneless as a memento of his visit.

Fluttershy – kindness
In It Ain't Easy Being Breezies, Fluttershy cares for some Breezies who are separated from the rest of their swarm during their home migration. However, her kindness is taken advantage of to the point at which the Breezies she cares for don't wish to leave. One of the Breezies named Seabreeze helps Fluttershy understand that being firm can be the greatest kindness one can show, while Fluttershy teaches him that kindness and team work can be helpful too. As thanks for helping him return home, Seabreeze gives Fluttershy a flower with blue petals.

Applejack – honesty
In Leap of Faith, Applejack becomes entangled in an attempt by Flim and Flam to sell a worthless vegetable tonic as a "cure all" medicine. When she realizes that her words of approval about the boost of confidence it had given Granny Smith were taken out of context as an endorsement of the tonic, she confesses to having unwittingly misled the ponies of Ponyville. In response to her honesty, Silver Shill, the brothers' accomplice, backs up her denunciation of the con artists. He then gives her the bit he took from a pony for a bottle of the tonic and promises to repay his customer from his own funds.