[arcticstorm]

When I read through this passage, Tom's tone of voice and wording almost sounds somber as if he is mourning for the loss of this person as if they were really close.
Assuming that the Barrow downs and with them the Barrow wights began their existence in the war that lasted from TA 1409- TA 1975, we can tell from the Adventures of Tom Bombadil that Tom didn't marry Goldberry until sometime after that point, because the Barrow-Wight attempted to "kill" him. Tom married Goldberry after this time. It would indeed be odd for him, being "alive" from the beginning, to be unmarried until about TA 2000.
Obviously, the person this brooch belonged to, is one that Tom highly admired, or even deeply loved,
"Goldberry will where it now, and we will not forget her"-- Tom Bombadil

If it is to be assumed that the Barrow-wight could heve killed Tom if he would have let him, what is saying that the Wight couldn't have killed Tom's former wife, if indeed he had one. Tom isn't omniscient , he needed Frodo to call for his help to even be able to come help the hobbits. if his former wife was wandering alone and taken by the wight to the barrow, she may not have been able to call for help, think about Sam, Merry, and Pippen. I do not think everything in the barrow was buried with him, maybe some of the treasure came from his victims.
Therefore, I think this brooch came from Tom's former wife, who is never mentioned or alluded to in the story but is a high possibility.