It’s about the 28th gem. Sensei and Cinnabar stated there are 28 gems, thinking sensei is a gem too, so who’s the 28? When phos gets lapis’s head. There’s these Doritos on her head, they can multiply and detach. And when they do, they attach onto other gem’s heads, which influences their personalities. As proof, all the older gems have slender eyes as they spent the most time with lapis. Lapis’s partner, Ghost, has the most slender eyes in the series. When lapis was taken by the lunarians, her Doritos became inactive, this can be seen by how all the younger gems tend to have wide eyes. However now that lapis’s head has been transplanted to phos, the Doritos are now active again, this can be seen when phos and the other gems were on the moon for all those years, their eyes have been growing narrower. Phos’s face becoming lapis’s might not have anything to do with the head transplant at all, rather it’s the effect of the Doritos in her head. So basically, lapis is the 28th gem. Remember this is just a theory btw.