Good try (and welcome to the 'Downs btw!), and I might also add that it surprises me that it took so long for somebody to try this answer, because I have been expecting it far more early. However, as you might have figured from my way of writing, this is not the answer I am looking for. Because, actually, the person who cast the Silmaril into a pit was Maedhros, not Maglor, but neither of the names is the correct answer for this question anyway. Because Maglor cast his Silmaril into the Sea, not into a pit, whereas Maedhros cast himself into the pit along with the Silmaril. And the person who I am talking about didn't cast itself in there, just the abovementioned
"his/her own greatest and most beloved treasure".
(I think it could also be argued whether the Silmaril could be called their "most beloved treasure" at the time.)
But do not let the unsuccessful attempt scare you away
It is nice to see "new blood" in the Quiz Room.