Necklace | Sailor Moon | Cosmic Heart Compact

The Cosmic Heart Compact is the 3rd transformation brooch given to Usagi Tsukino to transform into Sailor Moon. It was given to her in episode 91 along with the Spiral Heart Moon Rod. To transform, she shouts "Moon Cosmic Power, Make Up!". (from Sailor Moon Wiki)

Sailor Moon
A 14-year-old underachieving young schoolgirl named Usagi Tsukino meets a magical talking cat named Luna. Luna gives Usagi the ability to transform into a magical alter ego — Sailor Moon — tasked with locating the moon princess and battling the evil forces of the Dark Kingdom. When Usagi transforms for the first time into her magical sailor suit with Luna's help, she overreacts and reluctantly accepts her fate, not sure what has happened to her. At the time she does not know the enemies she will face, the friends she will make, or the experiences ahead of her. As she moves forward, she accepts her fate, and realizes the importance of fighting evil. [from Wikipedia]

Sailor Moon is the protagonist of the Sailor Moon series. Her civilian identity is Usagi Tsukino ("Serena" in the English dub, and "Bunny" in many other countries). She is the second Senshi to be awakened by the moon cats, and the only character to appear in every episode of the series. Over the course of the series, she grew from a frightened heroine unsure of herself and her abilities into a brave fighter for love and justice. (from Wikipedia)