While making the eyes for a character I was creating, I came across an issue where when I mirrored the first eyeball, the result was textured incorrectly.
As you can see in the picture above, the original eyeball (on the right) has sort of a gradient on the iris where the outer rim is a darker brown than the inner rim. But then, if you look at the mirrored eyeball (on the left), it completely loses the effect and just becomes what looks like a solid color. I have tried applying the mirror modifier but it does nothing.
This effect was 100% procedural and achieved through a node setup I found in a tutorial. I'll also share it here for convenience. If anyone is wondering, the ColorRamp actually has two colors except they are extremely close to each other. I can share the file as well if anyone needs it. The white and the pupil of the eyeball are separate materials.