The George VI Festoon Necklace. 

In 1950, King George VI had a diamond necklace created for his daughter Princess Elizabeth using 105 loose collets that were among the Crown heirlooms he inherited. (These, according to Hugh Roberts, had been used by Queen Mary to change the lengths of her multiple diamond collet necklaces, hence their loose status in the collection.) The end result is this take on a triple strand necklace: three strands of graduated collets suspended between two diamond triangles, with a single collet strand at the back. This is also called simply the Queen’s Festoon Necklace, though I’ll use George VI’s name to be a little more specific