Board Thread:General Discussion/@comment-27182540-20140406032026/@comment-4651179-20140504212936

Bridgetterocks wrote: Natasha's arrow-shaped necklace was Scarlett Johansson's idea

Yes, and this is what the Russo brothers said about it:

''Honestly it was a choice that Scarlett made as a character, and... her relationship with Hawkeye will become very clear in Avengers 2.''
 * --The Russo Brothers on The Empire Podcast