A "diamond" is forever

[Aiwendil]

Alatar wrote:

First, the Silmarils had to be indestructible as a plot devise. If they were perishable, then Feanor could have destroyed them to spite everyone.

Why on earth would he do that? Feanor loved the Silmarils more than anything else (the only possible exception I can think of being his father).

Actually, the Silmarils were destructible. After the darkening of Valinor the Valar asked that Feanor break the Silmarils and use the light trapped within to rekindle the Trees. And he refused. The point was moot because, unbeknownst to them at the time, Melkor had just attacked Formenos. But it remains true that even at the urging of the Valar, even to revive the Two Trees and reverse the Darkening, Feanor would not destroy the Silmarils.

The jewels came close to destruction again when Melkor quarreled with Ungoliant. Ungoliant desired to consume them but Melkor withheld them; this led to the Thieves' Quarrel. The arrival of the Balrogs prevented Ungoliant from consuming them, but the text (LQ2 I believe) clearly indicates that had Ungoliant consumed them they would have perished.

Finally, one may note that the Second Prophecy of Mandos foretells that at the end of the world the Silmarils will in fact be broken and their light will be used to revive the Trees.

So they were not indestructible. They were certainly difficult to damage or destroy, but not impossible.