Clara's leaf
Clara's leaf, dubbed "the most important leaf in human history", was a large red ovate leaf which caused the meeting of Dave Oswald and Ellie Ravenwood.
History
On a windy day in Blackpool in 1981, this leaf fell from its tree and flew into Dave's face. This led him to almost get hit by a car, before being saved by Ellie. After, they began dating and eventually got married. Dave kept the leaf as a symbol of their love.
Ellie put the leaf in the first page of her book, 101 Places to See, which she later passed on to her daughter, Clara Oswald.
Clara, who became a companion of the Eleventh Doctor, used that leaf to feed Akhaten, an entity that fed off peoples' souls, and therefore stories. The leaf represented the history of the family, as well as potential futures. The infinite amount of stories in that one leaf bloated Akhaten, and he imploded. The leaf was destroyed in the process. (TV: The Rings of Akhaten)
When Clara entered the Doctor's time stream on Trenzalore, the Eleventh Doctor was able to send her the leaf in order to guide her back into the world. (TV: The Name of the Doctor)
Clara Oswald had a maple leaf pressed in the pages of 101 Places to See. After saving Clara from having her mind being downloaded by The Great Intelligence, the Eleventh Doctor discovered the book containing the leaf. He asked Clara about and she told him, "That wasn't a leaf, that was page one." (TV: The Bells of Saint John)
Behind the scenes
It is unclear whether or not the maple leaf is the same as "the most important leaf in human history". It might however simply be a production error which caused the leaf to change form.
