I've been following the comic for a while now, since the gems didn't know what Steven was and everyone was trying to come up with reasons they would recognize him as WD. So I'm sure this must have been answered at some point, but I can't find it in the FAQ or with tumblr's awful search function, so I'm sorry for the repeat question. Why didn't they recognize him based on the diamond pupils, or at least wonder about them if they thought for sure he couldn't be a diamond?
Ah, I did remove that question from the FAQ at some point, so that’s on me.
Basically the reason was that Diamonds are not *only* defined by their pupils but also by their rarity and enormous size. Although his eyes are very specific, keep in mind that they don’t actually LOOK like White’s eyes at all:
She has white pupils, and a much lighter color.
On top of that, she’s gigantic, and looks and sounds nothing like Steven himself. Add that to the fact that Diamonds are not just ‘another type of gem’ and it is well known that there is a set amount of them... well... it was just a very big conclusion to jump to.
They did actually notice his eyes being ‘off’, however, and it’s addressed (albeit very shortly) here:
They attributed this fact to his off-color-ness and tried not to bring it up. I think I had a comic planned where Rose specifically addressed his eyes, and he replied with the usual ‘oh, I have coloboma’ schtick, which caused to her essentially go ‘oh, I have no idea what that means but he’s clearly shapeshifting to fit in with the humans’.
I don’t deny that it’s a bold narrative step to take on my part and may be counted as a plot hole, but personally I think it’s interesting to see how far we can stretch the plausibility of the gems just never questioning a Diamond being in their midst despite quite a few tells which, to us, the viewers, may appear obvious. ;)