A "diamond" is forever

[Amarie of the Vanyar]

True, I forgot Míriel's death [img]smilies/frown.gif[/img]

After reading your posts and considering her death, I agree with you in that it is likely that Fëanor made the Silmarills because he wanted to possess something that would never abandon him unlike his mother.

But in my opinion, it is impossible that Fëanor knew the meaning of words like rust or decompose when he made the Silmarills, because he was born in Aman and there:

For in Aman no creatures suffered any sickness or disorder of their natures; nor was there any decay or ageing more swift than the slow ageing of Arda itself. So that all things coming at last to fullness of form and virtue remained in that state, blissfully, ageing and wearying of their life and being no swifter than the Valar themselves.
'Myths Transformed', HoME 10 'Morgoth's Ring'

Even Míriel's hröa didn't rot when her fëa went to Mandos. [img]smilies/frown.gif[/img]