He was using Fiendfyre to kill Harry by it had gone too far (basilisk venom and Fiendfyre are the only things that destroy Horcruxes) and destroyed the diadem and it killed himself.
They have to destroy four more (Nagini, the goblet, diadem, and Harry).
In the Room of Requirement
A diadem is a jeweled crown worn as a symbol of sovereignty. The diadem in the Harry Potter books is Rowena Ravenclaw's and it was believed that it bestowed apon the wearer incredible wit. It was stolen by her daughter a long time ago and was lost. This is why the diadem is refered to as "the lost diadem of Ravenclaw" In the last part of the book, Harry and his friends believe it is a horcrux and contains a piece of Lord Voldemorts soul. Voldemort cannot die without all the horcruxes being destroyed so therefore, Harry needs to destroy the diadem. The only problem is that it is lost.
The diadem was one of Tom Riddle's Horcruxes. A diadem is like a sort of crown.
Ron desroyed Salizar Slytherin's locket using the Sword of Griffindor. Snape's doe patronus led Harry to the sword and Ron retrieved the sword while rescuing Harry.
the ring, the diadem,harry potter, voldemorts snake, the diary, a cup
The answer I believe is 'diadem'. In Harry potter, they are in search of ravenclaws lost diadem and it turns out to be a tiara
He didn't - Hermione did. When Harry is searching for Ravenclaw's diadem Ron and Hermione go down to the Chamber of Secrets to get some Basilisk fangs (one of the few things that is able to kill a Horcrux). Hermione then stabs the cup and destroys it.
how do i know ask some one else
He had doubts at one point because the diadem had supposedly been lost centuries before Voldemort's time.
Vincent Crabbe destroyed the diadem when he used fiendfyre (cursed fire) on it, while trying to kill Harry Potter (along with Ron Weasley and Hermione Granger).