Still reading The History of the Hobbit by John D. Rateliff. There have been loony theories around the Internet which ponder whether the Arkenstone from The Hobbit is in fact a Silmaril.

Interestingly, while the Arkenstone may not actually be a Silmaril, it shares a lot of characteristics with the holy jewels, and these similarities are unlikely to be an accident, since Tolkien was working on The Hobbit simultaneously as he continued to elucidate the stories of his legendarium.

And in Tolkien’s version of the Quenta written in Old English, he writes the following (translated by Ratecliff):

Here through cunning craft/artistic skill the Noldor elves devised and created many gems, and Fëanor the Noldor lord wrought the Silmarils, that were holy stones: Eorclanstánas or Arkenstones.