A "diamond" is forever

[Manardariel]

It is possible that Feanor wanted to create something undestructable, something that would not be broken. However; I agree with Amarie that Feanor really wasn´t familiar with death, or things fading away. So his motivation wasn´t to create something that wouldn´t die, but something that couldn´t be destroyed. That says a lot about a character, doesn´t it? He´s not nescesarily thinking of the natrual end of a thing, but of making something that couldn´t be destroyed by force.
And that fits Feanor, who didn´t want anything he made be destroyed, and who opposed forces greater than him.