The jewel of highest value to the dwarves of Middle-earth. In J.R.R Tolkien's "The Hobbit", the Arkenstone was found by Bilbo Baggins and he took it and traded it to the men near Erebor. In doing so, he incurred the wrath of Thorin and his dwarves, who nearly took Bilbo's head off. Also called the "Mountain's Heart."
"Man, that jewel is almost as beautiful as the Arkenstone!"