Based on Maridje's photo I think it might be Queen Victoria's Diamond plaques/squares.

It has the squareness and the smaller linking elements.

I think the dark parts are not sapphires but reflection form HM's dark blue dress.

It is so hard to tell with these older pictures.

I am 100% sure it is not the (current) three sapphire bracelet made from the Loop elements when the necklace was modified.