For what it's worth, the peerless quality of the Arkenstone could be precisely
because the Dwarves cut it. When Bilbo recognises that there could not be two such stones in the whole world, he might be noticing the flawless gem-cutting craftsmanship of the Dwarves as well as the enormous size of the flawless jewel. This would be congruent with Tolkien's general presentation of Middle-earth as beset with the "long decline" from original greatness into later imitation. It would be entirely consistent for the Dwarves to never again be capable of cutting so well another Arkenstone--even if one ever turned up again to be cut. The Dwarves of Thorin and Dáin's day were no longer the Dwarves who had lived in Moria (as they would have been in Thráin I's day) or even those of pre-Smaug Erebor. The Kingdom Under the Mountain would flourish again... but it would not be the Golden Age.
Also, as a somewhat impish aside, the comment that there could not be two such things in the world need not indicate at all that the Arkenstone is the
greatest or
most beautiful gem in the world... though it is clear from the text that the Arkenstone wasn't the ugliest gem in the world...
