A mystery within a mystery, involving Tom Bombadil.

[Fingolfin II]

I also don't own a copy of The Adventures of Tom Bombadil, but my guess is that he is touched by the brooch's beauty, and that it could be a symbol of the beauty of the environment before the Barrow-wights and 'sinister' trees (i.e. Old Man Willow) moved into the Barrow-Downs.

'Here is a pretty toy for Tom and his for his lady! Fair was she who long ago wore this on her shoulder. Goldberry shall wear it now, and we will not forget her!

This suggest to me that this was worn by some beautiful Elven lady in the First Age, possibly even Luthien or her Maia mother, Melian. As Goldberry is described as fair as 'a young elf-queen clad in living flowers', no doubt the beauty of probably a dead, or gone, Elf lady who wore this long ago brought to mind Tom's beautiful mistress, Goldberry. So I think this is probably a better answer than the one above, and that he remembered not only the beautiful Elf lady long ago, but all Elves that have perished and gave it to Goldberry in memoriam of them.