Just so you know in advance, this is perhaps my favorite of Queen Elizabeth's brooches: the Williamson Diamond Brooch.
This brooch is all about its center stone: a 23.6 carat pink diamond - the finest pink diamond in existence, according to the Royal Collection. It comes from Tanzania, where it was discovered by a group of children under a tree outside a mine owned by Dr. John Williamson. Dr. Williamson, a Canadian geologist, decided to give the diamond to Princess Elizabeth as a wedding gift in 1947. The stone was cut down from its original rough weight of 54 carats into a round brilliant the following year in London, and a new brooch in the form of a jonquil was made by Cartier to complement the diamond in 1953 (using diamonds also provided by Williamson).
Receiving the pink diamond with (a slightly envious, perhaps?) Queen Mary
This pretty thing is one of the queen's favorites. She wore it for her Silver Jubilee celebrations in 1977 as well as for the weddings of Charles and Diana in 1981 and Edward and Sophie in 1999.
There's a very sweet story involved with this brooch: one of the children that Dr. Williamson asked to bless the original stone and wish the princess luck before sending it off grew up and earned himself the honor of an MBE and an invitation to an investiture with the queen. He wrote to the queen in advance of his ceremony and told her his story; on the big day, she wore the brooch just for him.
In action on Silver Jubilee day
This is one of the pieces that will be on display in "The Queen's Diamonds" exhibit this summer at Buckingham Palace - surely a must see if you're lucky enough to be in the London vicinity.
