We all know about the Elements of Harmony (under E, BTW) and that the Mane Six represent each of the six elements; Twilight: Magic; Rainbow Dash: Loyalty; Applejack: Honesty; Pinkie Pie: Laughter; Fluttershy: Kindness; and Rarity: Generosity. But what about Starlight Glimmer? Recently, Starlight Glimmer joined the Mane Six, but she has yet to receive her own place on the throne in Twilight's castle or her own element. So the question is, what is Starlight's Element? If I had to guess, I would say that Starlight Glimmer is the Element of Peace.
To prove my theory, let's go back to Starlight's childhood and her friendship with Sunburst. Starlight and Sunburst were extremely close. Starlight herself said that she and him were barely ever apart. But then when Sunburst got his cutie mark, he was sent away and Starlight didn't see him for a long time. She was so heartbroken and hurt, she became embittered and blamed cutie marks for her losing her friend. After that, she never made another friend, fearing that a cutie mark would take them away just Sunburst's did.
Over the years, she remained angry and embittered, studying magic, never making a single friend, until she discovered a spell to take away cutie marks. After learning magic and leaving her home, her travels took her to Our Town where she established an anti-cutie mark cult, taking away ponies cutie marks, convincing them that cutie marks and individuality were bad. For a while, she seemed happy, but then after Twilight and her friends exposed her secret and turned the town against her, she ran away and swore revenge.
Eventually, she discovered a time traveling spell and used it to sabotage the day when Rainbow Dash performed the Sonic Rainboom that brought the Mane Six together to get back at Twilight.
After realizing she couldn't be stopped, Twilight tried to convince her to stop, showing her the consequences of her actions. She then shows Twilight her past, growing angry and sad in the process. After some convincing, Starlight is convinced to stop her plan of revenge and becomes Twilight's pupil.
In the Season 6 pilot, Starlight reunites with Sunburst and after some talking, laments about her past and eventually makes peace with him.
Later in the season 6 finale, Starlight makes peace with her past in the village after helping defeat Crysalis.
Starlight from her childhood endured a massive heartache which molded into the villain she was introduced as, but through Twilight's teachings (and it makes me sick to my stomach to give Twilight credit for anything), she was able to reconcile with him. Not only that, but before that, she made amends with ponies of her village. And finally, she made peace with herself as a stepping up to be a leader.
The main essentials of Starlight's character arch has been her coming to terms with her past and making peace with those around her. She represents the notion that even if you have something from your past that has hurt you for the longest time, with enough strength, you can overcome it and make peace with it.
And peace is just an important factor in friendship as anything else. I'm there are some of us out there who have a friend that we haven't talked to in a long time either because of a girl or they did something bad they did or something bad you did and it's not easy to just make amends. Starlight is the person/pony we all wish we could be, someone who overcomes past discretions and makes peace with them.
And that is why Starlight Glimmer is the Element of Peace.