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 recieved a new one in Act 4. And in Act 8 Sailor Moon's tiara changed shape, when that one breaks and Princess Serenity is reviled her tiara is replaced by a crescent moon on her forehead.
Sailor Jupiter's tiara has an antennea which she uses like a lightning rod.
When we first see Sailor Venus she has a crescent moon on her forehead then it is replaced by a tiara.