[Son of Númenor]

I think Melkor's and Ungoliant's lust for the Silmarils has everything to do with the nature of evil - specifically, the desire to own & to corrupt all that is 'good'. Melkor desires omnipotence, essentially in the form of rulership over and corruption of all that was once Eru's (&, thus, the Valar's - the Light of the Trees, the Silmarils, etc.).