As far as I know, there's no explicit answer in canon. Sauron's state of mind at the time he forges the One isn't really given any treatment in the text.
One might argue that gems are too closely connected to the Elves for Sauron's taste; we're told in The Silmarillion that they discovered the gems that existed beneath the earth:
[I]t came to pass that the masons of the house of Finwë, quarrying in the hills after stone (for they delighted in the building of high towers), first discovered the earth-gems, and brought them forth in countless myriads; and they devised tools for the cutting and shaping of gems, and carved them in many forms.
The Silmarillion III Quenta Silmarillion Chapter 5: "Of Eldamar and the Princes of the Eldalië"
And that they invented the process of constructing gems:
The Noldor also it was who first achieved the making of gems; and the fairest of an gems were the Silmarils, and they are lost.
The Silmarillion III Quenta Silmarillion Chapter 1: "Of the Beginning of Days"
It may also have been a form of rebellion against his former Valar master Aulë, most skilled of all craftsmen. How better to show a craftsmen that you've abandoned them, then by enslaving the world with something so plain and (aesthetically) uninteresting?1.
Another possible theory is suggested (obliquely) by the essay "Notes on Motives". In it, Tolkien suggests that it was the lingering strains of Morgoth's power (especially present in gold) that allowed Sauron's arts in the first place:
Sauron's power was not (for example) in gold as such, but in a particular form or shape made of a particular portion of total gold. Morgoth's power was disseminated throughout Gold, if nowhere absolute (for he did not create Gold) it was nowhere absent. (It was this Morgoth-element in matter, indeed, which was a prerequisite for such 'magic' and other evils as Sauron practised with it and upon it.)
History of Middle-earth X Morgoth's Ring Part 5: "Myths Transformed" Chapter VII "Notes on motives in the Silmarillion" (ii)
This would suggest, then, that the actual form of the Ring was crucial to its proper function; if a gem had been included, its form would have been disrupted and its power not precisely what Sauron intended. It may also be that, by keeping the Ring purely of gold, Sauron didn't dilute the "Morgoth-element", contributing to the Ring's magical power.
I've been looking through Tolkien's Letters, and found nothing discussing any symbolism surrounding the Ring. I personally rather like the theory put forth by milesper in a comment on the question:
Seems to me like its symbolic, a choice made by the author. There plainest, most humble of the rings is the most powerful—kind of like hobbits.
But I've as yet found no evidence to support it.
1 Well, I guess you could wage a millenia-long war against the Gods. But that sounds exhausting