Anniversary charm pendants in solid 14K and 10K gold. Many are engravable on the back.
Year charms. Numbered milestone designs — the piece bought at a tenth, a twenty-fifth, a fiftieth.
Entwined designs. Interlocking hearts and linked rings that say the thing without a number on them, which suits an anniversary that is not a round one.
Engraving. A date on the back is what turns an anniversary charm into a record. It is also what makes it hard to replace, so worth checking the spelling before it is cut.
See also heart pendants and charms, number charms for building a year, family charm pendants and all Charm Pendants.