Yes, lusting for a Silmaril was a dangerous thing (Thingol). The Silmarils, like Rider said, contained the last of the two trees, that had created bliss in Valinor. Plus, just the belief that they belonged to the Elves, specifically the Sons of Feanor.
I don't really think the Silmarils had a particular power, but they shone of their own light and only through them could the Two Trees be rekindled.
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