I think that the "Elaborate" Necklace that Knightship highlighted above is QA's Wedding Necklace with "bits and bobs" added to it.

The Wedding Necklace itself consists of the 8 domed pearls encircled with diamonds joined by the double swags. It also includes the 3 smaller drop pearls hanging from the 3 central clusters.

At first, I thought the large central pearl drops were the drops from QA's Wedding Brooch - but they are not big enough.

I think that Eiriksn is correct that the large pearl drops are from the Dagmar Necklace.

Andrew U picked up on some other additions - the diamond chandelier style drops on either side of the central element that are now worn by Queen Sonia Of Norway.

The outer pearl drops may be from QA's Wedding Brooch.

It also looks like QA has incorporated a paid of diamond drop earrings attached to the swags between the pearl drops.

I think OmerH was correct that the central element was part of the parliament stomacher - but I think just the lower part.

I think the upper part of the parliament stomacher is actually the large diamond brooch worn on QA's right shoulder between the 2 star brooches. I an not sure of the origin of this brooch.

p.s. in her son's wedding photo, QA also wears her Wedding Brooch central on her sash sans pearl drops.

All in all - quite the impressive pieced-together decolletage !!