From The Windsor Diaries by Alanthea Fitzalan Howard who spent her war years in close proximity to the Princesses and their parents while they were at Windsor Castle.
On the occasion of Princess Elizabeth’s 16th birthday, Alanthea wrote
Tuesday 21 April 1942 -
"I was shown L’s (Lilibet’s) presents; she had two lovely diamond stars for her hair from the K.(ing) and Q.(ueen)”
In the 1930s the Duchess of York, later Queen Elizabeth, wore diamond stars in her hair on some occasions, such as visiting the ballet or the opera. In 1937, newspapers noted the Duchess wearing the gems.
"The Queen was in white and wore diamond stars in her hair and diamonds earrings."
".....The Queen was in white and wore diamond stars in her hair and diamond earrings
".....the Queen had diamond stars set most becomingly over her left ear and looked extremely well in soft white with a fluffy ostrich-feather wrap-also in white. "
The late-Victorian diamond star brooch, in the picture above, has eight rays and a collet diamond on a knife-wire between each of its eight points. A second star brooch with six rays and six collet diamonds is known and in the possession of Princess Anne. She wore it at for the first time publicly on the state visit at Austria in 1969, when she was 18. It may have been one of the pair of diamond stars head jewels of Queen Elizabeth's 16th birthday gift from her parents.
Special thanks to Laura, who sent me the important text from the book.
Sources: The Windsor Diaries by Alanthea Fitzalan Howard; Belfast Telegraph; Edinburgh Evening News; Life Magazine; The Tatler and Bystander in Town and Country Birthday Ceremonial;