I feel like Bilbo would teach the elves of Rivendell the concept of a mathom with the express purpose of oh so politely asking them over dinner how were the Silmarils not a mathom
Random Feanorian elf inherited by Elrond: That’s not… No.
Bilbo: Oh so they were useful?
Elf: Well. No, not exactly
Bilbo: But the owners didn’t want to throw them away.
Elf: Yeah
Bilbo: And they were in possession of several people over the course of years.
Elf: Yeah
Bilbo: That’s a mathom.
Elf: No! We loved them because were very pretty!!! And one of a kind!!! Crafted by hands more skilled than any of ours!!!
Bilbo: Yes, like my great aunt’s set of painted dishcloths
Elf, in tears: The Silmarils were not like your great aunt’s set of painted dishcloths
Bilbo: How’s The Great Mathom War as a title for a poem about the First Age?
By the way, it’s important to me that Elrond supports Bilbo’s claim and finds it quite insightful actually