So each of the other great rings had a gem stone in it. And the lesser rings were "round and anadorned""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."
The Tolkien Companion describes Narya, Gandalf's ring, as being adorned by a single stone as 'red as fire.' It also says that Vilya, Elrond's ring has a great blue stone.Its rays glanced upon a ring about her finger; it glittered like polished gold overlaid with silver light, and a white stone in it twinkled as if the Even-star had come down to rest upon her hand.