New evidence for the Arkenstone-Silmaril case

[Morsul the Dark]

Originally Posted by William Cloud Hicklin

Tolkien tells us that Feanor made the substance of the Silmarils "more strong than adamant, so that no violence could mar or break it within the Kingdom of Arda." But we are told of the Arkenstone that "the Dwarves had cut and shaped it into myriad facets."

Where are you getting that last quote? I’m searching but can’t find a source for its making.