First appearance: Act 1

Luna gives Usagi this necklace in the first episode. If Usagi shouts "Moon Prism Power!" when wearing it, it transforms into the Heart Moon Brooch (a bigger version of the heart on the necklace) and she begins her transformation into Sailor Moon. Sailor Moon wears the Heart Moon Brooch on her chest.

Anime/Manga Comparison:
In every story arc of the anime and manga, Usagi gets a new compact that she uses to transform. Whenever she's in her school uniform, she wears her compact on her chest as a brooch, just like when she's Sailor Moon. On the outside, the Heart Moon Brooch from the live action series looks a lot like the third compact from the anime/manga, the Cosmic Heart Compact.