The One Ring: A Thing of Beauty?

[Morwen]

"The Nine, the Seven, and the Three," he said, "had each their proper gem. Not so the One. It was round and unadorned, as it were one of the lesser rings; but its maker set marks upon it that the skilled, maybe, could still see and read." (my emphasis)

Gandalf recalling the words of Saruman, FotR, The Council of Elrond

It would seem that the One is purposefully plain so that a casual observer would not identify it as one of the great rings.

Originally posted by Sardy

Surely in crafting his accomplishment to beat all accomplishments, a little more flair might've been in order?

Well it is the One Ring to rule them all. The accomplishment IMO is that it exists. Let the Elves adorn their rings with whatever gems they liked. The One was still more powerful.
Besides I don't know that lack of a gem stone would make the One less beautiful. It is I suppose a matter of taste. There is a beauty in its simplicity after all, a perfect circle, "it looked to be made of pure and solid gold".