J.R.R. Tolkien, the Hobbit: But fairest of all was the great white gem, which the dwarves had found beneath the roots of the Mountain, the Heart of the Mountain, the Arkenstone of Thrain.
"The Arkenstone! The Arkenstone!" murmured Thorin in the dark, half dreaming with his chin upon his knees. "It was like a globe with a thousand facets; it shone like silver in the firelight, like water in the sun, like snow under the stars, like rain upon the Moon!"
Later Bilbo Baggins steals the Arkenstone from the treasures of Smaug the dragon, and uses it to force negotiation between Thorin Oakenshield and Men of Dale.
It seems Arkenstone had some bewitching attraction, just like the Silmarils and other magical jewelry.