When princess Clestia and princess Luna turned Discord to stone, the Elements backfired and they were turned into stone orbs.
The Element of Magic was not found until Twilight came along
The 2012 trading cards of My Little Pony, mention a Seventh Element of Harmony given to Princess Cadance, Love.
This is not canon, this is the truth. I've seen it. It might even possibly crop up in season three.
Princess Cadance had to wearing her element when they defeated Chrysalis, so I think it’s her crown! The Seventh Element exists, the Element of Love is coloured green because there is a green element shown in the first episode: Friendship is Magic Part 1, however there is no green Element used by the mane 6 or anywhere else in the entire series. It is also coloured green to match the colour of Queen Chrysalis, who feeds off love.
Mystery solved? Please note this is only my personal theory, but could actually be viable in the show.