Board Thread:Fun and Games/@comment-29979363-20170120203844/@comment-25101089-20170120224540

I would say no, and yes.

It can't be a Silmaril because it seems very unlikely that a Silmaril thrown into a chasm in Beleriand, a mere 6000-ish years later, would somehow find its way into a volcano far to the East. Also, as you say, the Silmarils burned people, and the Arkenstone does not. Tolkien also wrote that the Silmarils would remain lost until the end of the world.

But from another perspective: Yes, it is a Silmaril.

Because when Tolkien originally wrote The Hobbit, he did not intend for it to be set in the same world as his Silmarillion legends. In fact, it was only once he started writing the sequel, The Lord of the Rings, that he decided to set it in the same world as the Silmarillion, and have the Silmarillion legendarium present as backstory in the world. He then changed parts of The Hobbit in later editions to make it fit better into this world, but it was never intended to be set there originally.

Parts of The Hobbit are very clearly taken from the Silmarillion, such as Mirkwood - a corrupted forest full of evil creatures, very similar to the Taur-nu-Fuin in Dorthonion (Taur-nu-Fuin even means Forest-under-Shadow, or in other words - Mirkwood) and the Elvenking Thranduil in his underground halls within the Woodland Realm, very similar to the Elvenking Thingol in his underground halls of Menegroth within the woodland realm of Doriath. There are the Necromancer in the Hobbit, and the Necromancer Sauron in the Silmarillion, who were not originally supposed to be the same. Neither were Elrond in the Hobbit and the Elrond of the Silmarillion. So when Tolkien wrote about the Arkenstone, he was almost certainly thinking of a Silmaril - reusing an idea from his previous work to put in this (originally) unrelated fairy-story.

http://www.tolkiendil.com/essais/tolkien_1892-2012/john_d_rateliff