[the guy who be short]

Of dwarves in Nogrod and Belegost trading with Doriath and building king Thingol's halls:

And though the dwarves ever demanded a price for all that they did, whether with delight or with toil, at this time they held themselves paid. For Melian taught them much wisdom, which they were eager to get; whereas thingol rewarded them with many fair pearls...The Naugrim had not before seen their like, and they held them dear. And one there was great as a dove's egg, and its sheen was as the starlight upon the foam of the sea; Nimphelos it was named, and the chieftan of the Enfeng prized it above a mountain of wealth.

HoME XI, The War of the Jewels, p11
That's all we have describing this opal. I cant remember how the Arkenstone was found in the Hobbit, does it say? Because perhaps the refugess from Nogrod and Belegost brought it back to Moria, and those dwarves took it to Erebor, then Erebor was attacked.