The Marchionesses of Londonderry seem to have worn not only the famous pearls seen in the photos Janet posted (those purchased in Vienna in 1821) but also a separate pearl necklace with large pear shaped pearl drops.
Photo posted by Janet
When Edith, as Lady Castlereagh, presented her sister-in-law at Court in March 1907 she wore the pearl tiara with the upright pear shaped pearls as well as a different necklace.
Detail
Later, as Marchioness of Londonderry, Edith wore the necklace to the Opening of Parliament in November 1932
She also wore the necklace, but I think with the larger pear shaped pearl drops, to the 1937 Coronation.
1937 Coronation.
How often the Marchionesses wore these pearls is difficult to say, as there are few photos publicly available; however, Edith, as Dowager Marchioness, was photographed wearing the tiara without the upright pear shaped pearls and the earrings in 1954.
1954
In the 1954 photo above I think that the large bracelet is the same as the one in photo of Edith, Marchioness of Londonderry wearing the large pink topaz brooch in 1952.
1952