The text on that page in The Art of Star Trek
Though committed to the idealism of the twenty-third century, Gene Roddenberry was not blind to the financial realities of the twentieth. Realizing the demand among viewers for merchandise from the STAR TREK universe, Roddenberry worked with William Ware Theiss to create a symbol of Vulcan philosophy, which could also be sold to fans. These sketches by Theiss show the development of two possible STAR TREK medallions which culminated in the IDIC--an acronym for Infinite Diversity in Infinite Combinations. The IDIC was first worn by Spock in the episode "Is There in Truth No Beauty?" and has since been seen as a recurring Vulcan symbol in current STAR TREK productions.
So I think the most likely scenario is that Theiss designed the medallion under Roddenberry's guidance -- the standard relationship between art staffer and showrunner where the showrunner tells the artist what he's looking for, the artist comes up with various suggestions, the showrunner picks the elements he likes, and back and forth until a final design is reached.
Then again, that first pencil sketch (actually the last drawing on the TAoST page) seems to be in a cruder hand than one would expect of Theiss, and I'm not sure the handwriting matches Theiss's in the costume sketches reproduced elsewhere in the book. So that initial sketch could've been Roddenberry's. But the caption includes it among the "sketches by Theiss." Harvey
, do you have copies of any documents with Roddenberry's handwriting on them for comparison purposes?
Anyway, I suppose I would credit it to Roddenberry and Theiss together, with the proviso that GR's involvement may have been no greater than his input into the design of the Enterprise
or any of its sets, costumes, props, etc. Which is to say, he would've always had input and approval into the designs, but he usually isn't given credit for it.