The Spencer tiara belongs to the Spencer family, and it was worn by Victoria Lockwood at her marriage to Diana's brother, the present Earl Spencer. The tiara was a bone of contention between the Princess of Wales and her brother during one of their periods of difficulty, as he asked for its return (I believe it is rightfully his). The other tiara worn by Diana as Princess of Wales, the Cambridge Love Knot, belongs to the Crown, and she was not entitled to use it after her divorce.
The Spencer tiara was left to Charles after the death of his grandfather in 1975. His sisters borrowed it from time to time until Diana married, at which time she used it frequently. After the separation, Lord Spencer asked Diana to return it.
The Cambridge Love Knot Tiara was a gift from The Queen and Diana retained it after the divorce. After her death, it was returned, along with Queen Mary's emerald choker and a few other pieces lent by The Queen. They will likely be given to Prince William's wife when he marries.