Although no one seems to have mentioned it, I believe that Queen Alexandra was forced to hand over Queen Victoria's regal circlet to Queen Mary and as a result had her own made which she wore in the 1913 photo with Queen Victoria's small diamond crown. QA's circlet, as Boffer pointed out, had a largish diamond in the front cross. Queen Victoria's did not. http://www.royalcollection.org.uk/collection/2106374/portrait-photograph-of-queen-alexandra-1844-1925-1913
Pope Hennessey noted in his biography of Queen Mary that there had been some dispute about jewels between QA and QM. QA, on the advice of her sister the Dowager Empress of Russia, argued that a Queen Dowager had precedence and so was entitled to keep the jewels; however, in reality this was not the English practice.