I have never seen any information about the origin of the long rope of pearls Queen Alexandra wore to the Opening of Parliament in 1901. They seem to have been regularly worn by the Queen on various State occasions and were described in the press. Some commentators referred to them as the Hanoverian pearls, but obviously this is not accurate.
It is possible that the necklace may have been a gift from one of the Princely families of India; equally, Queen Alexandra may have had pearls (and other stones) in her possession strung in the Indian style.
The earliest photo I have of Queen Alexandra wearing this necklace is of her at the Devonshire House Ball. In this the strand looks slightly shorter. I think that is because the necklace is draped behind the Queen's elaborate ruff collar and thenlooped over brooches at her waist.