The "False Tiara" is an item worn by Princess Sailor Venus in Pretty Guardian Sailor Moon when she masqueraded as the Moon Princess. The tiara first appeared in Act 12, when Sailor V showed herself as the Princess.
Although the Sailor Guardians and the Dark Kingdom assumed the Silver Crystal was set inside it, in Act 20, Sailor Venus revealed that she had several identical fake tiaras stored in a case after. After Nephrite seized it, she activated the tiara, causing it to explode, saying she had no plans to easily relinquish the true crystal.
All of her tiaras were fakes intended to distract the Dark Kingdom, and she stopping using them once Princess Serenity truly awakened, them having served their purpose.