This is the Duchess of Teck's Collet Necklace, in it's longest setting.
It was later shortened considerably.
Interestingly the Duchess of Teck's collet necklace was not left to the Queen Mother as stated in Roberts. It was left to The Queen; however in 1953, the Queen Mother wrote to her daughter asking if she could borrow some diamond necklaces to wear to the coronation, which The Queen gladly agreed to, yet these necklaces were simply not returned until the Queen Mother's death in 2002.
Information taken from a letter published in" William Shawcross, Counting One's Blessings: The Selected Letters of Queen Elizabeth the Queen Mother (2012)
--Previous Message-- : : Thanks Boris. I now see the sudden change : in diamond size. : : : : --Previous Message-- : I think Queen Victoria's necklace was : elongated by a seperate string of smaller : diamonds in the back. : The diamonds of the front necklace don't : graduate smoothly; at shoulder hight you can : actually detect the transition point with : the sudden jump in size (to smaller) where : the added back string is connected. : :