Queen Adelaide had a new crown made because the Crown of Mary of Modena was considered out of fashion (before Queen Adelaide, the previous coronation of a Queen Consort was the one of Queen Charlotte in 1760... 70 years before!). And Queen Alexandra had a new crown made because Queen Adelaide's crown had been dismounted from all its stones.
More recently, Queen Mary and Queen Elizabeth had a new crown made, because their predecessors as Queen Consorts (respectively Queen Alexandra and Queen Mary) were still alive at the time of the Coronation.
In this regard, I wonder what will happen when the Prince of Wales will be crowned: there will then be no living dowager Queen Consort, so Queen Mary's or Queen Elizabeth's crowns will be available, and Camilla could theoretically choose to wear either of them (provided she is crowned, of course, which is not certain, considering the status of her marriage to Prince Charles as regards to the law of the Church of England).