Sailor Moon's first tiara was given to her when she first transformed into Sailor Moon and it served as her first weapon. When removed from Sailor Moon's forehead and activated for her attack the tiara morphs into a flat circular shape with the gem placed on top. She received a new one in Act 4. In Act 8 Sailor Moon's tiara changed shape, when that one broke Princess Serenity is reviled and Sailor Moon's tiara is replaced by a crescent moon on her forehead.
Sailor Jupiter's tiara has an antenna that can retract in side. that she uses like a lightning rod to collect and store lightning until needed.
Sailor Venus' crescent moon on her forehead is later replaced by a tiara.