- A "blade of regret" - a steel weapon that was used in some way that the user later regretted.
- Filings from the wedding ring of a bride who lost their husband on their wedding night.
- A Rowan twig that has been gathered on Maundy Thursday.
- A pea that has been grown in the eye-socket of a human skull.
- A headpiece worn by a baby that was baptized in the same church that you were.
- A horseshoe from a horse that has thrown its rider, killing him.
The idea is that it's not that the item itself is innately valuable; no 500 g.p. gems to be crushed up or anything, but that the circumstances of its creation are so rare that an ordinary item becomes valuable due to its provenance. I kinda like that idea as a mechanism for keeping some more powerful spells rarely used, even though they might end up in someone's spell book.