This is certainly one of the best manga I've ever read and it is certainly the work with the best growth of the protagonist (mental and physical compared).
The author has been able to outline a growth of Phos step by step slowly and not losing any piece, but we must admit that this occupies a large part of the manga and that it leaves little room for other developments (some rare cases could be Dia or Rutile,which are not that much developed anyway).
One of the other flaws of this manga is perhaps the beginning that presents a slow style and it is not very clear where it goes and, which, however, I personally appreciated the fact that they do not reveal the feelings of the characters but it is you who owe them guess what obviously requires a not indifferent immersion in the work.
Phos at the begin :
The Phosphophyllite of the first 19 chapter can be considered to all intents and purposes a child who clings to the Sensei by completely relying not only on him but also on the other gems, resulting carefree, without judgment and willing to become useful for others.
(Basically the description of a little child).
Phos (new legs):
it does not show great character changes, except perhaps a greater sensitivity than before.
Phos (hair cut, ice floes and new arms):
Here, on the other hand, the character evolves incredibly, generating the first doubts after meeting the ice floes, and later a great sadness due to the first loss of a partner(of which he will begin to have visions), which marks her a lot by removing her smile and naivety, leaving room for a perennially melancholy look, even when he smiles.After these events the gems begin to respect it more and to assign it more and more tasks given the extreme utility and strength acquired with the new arms that make it UNIQUE.
Phos(new head):
And it is here that we have the most emblematic (physically) and sudden (psychologically) change, namely the transplant of a new head to replace the old one that has been lost.
The head in question is that of Lapis Lazuli, a gem previously brought to the Moon, this operation lasts 102 years and brings a radical change to Phos, giving it attention to detail and an intelligence typical of the old Lapis.He begins to express more and more his doubt towards Kongo until he reaches a point where he sacrifices certain things in order to discover the truth (for example the panzienza taught him by Padparadsca).
After learning about Morg's passing and Goshe saddens for them, he then becomes an example to follow for the new Morg and Goshe.
Phos at the end:
Towards the end we have a total loss of sense on the part of Phos, his will to help others has turned into an obsession for Kongo, so much so that he wants to destroy him definitively by blaming him for all his misfortunes, then after an inner reflection he will begin to hate all the gems remembering how in the past they denigrated her (clear symbol of her transformation into human).
A Phos that abandoned the state of apparent apathy and carelessness of the gems (probably given by immortality) becomes jealous suspicious etc ..... acquiring all the old human defects.The most interesting part in my opinion is seeing how in the end she herself became like the person she hated most in the world seeing and suffering everything he had experienced in his very long life.
Despite all the hatred she has attracted to herself, there are still people who cannot hate her like Euclase who we could define as the Pacifist compared to all the other gems, and she too wanted to find a solution to the problem, thus empathizing a little with Phos.
Or Kongo who loves her despite everything and would like to help her feeling an enormous sense of guilt towards her, and finally the last person among the "enemies" who could not hate Phos (and in my opinion the most interesting) is Jade who at shadow of Euclase wonders about Phos's motivations and wonders what happiness was for her, finding no answer and eventually asking her to apologize for never having understood her, for never having understood her pain.
SPEAKING ABOUT DIAMOND:
Dia at the begin:

I stop to talk about Dia because in my opinion she too suppur shown less than Phos is a great character who introduced in the very first chapters of the manga had already shown potential for growth, and so it was in fact from the beginning she turns out to be upset because of the his partner Bort, too strong too serious / severe.
She loves and respects Bort but there is something inside her that makes her feel bad when she is with her and what is that something?
The answer is: the sense of inferiority she feels in diminishing herself in comparison to Bort who seems to be perfect in everything.
Dia at the end:
Speaking instead of the story, you are given some bases and those are respected without going out of them too much, still managing to create an unexpected ending, this setting of the situation had given me to think a lot at the beginning and made me quite skeptical as the protagonists want just that living in peace with the People of the Moon and therefore I did not find a promise so greedy as to entice me to continue it, but you want the drawings or you want the protagonist himself I was captured by this story that has an incredible linearity and smoothness, 95 chapters literally flown .
(Spoier Allert)
THE END:

Many may have been banned from such an open ending in fact the mangaka would have wanted to conclude with chapter 98 and not with 95 and this is really a great shame because I admit that I would have been curious to read the ending thought by the author, but it is also It is true that not all evil comes to harm because if you consider this as an open ending you can travel with your imagination and think about what will happen when after 10 000 thousand years Phos wakes up:
These are some theories but if they can do many others so perhaps not always an open ending can only be seen as a defect as in the case of Houseki no Kuni.
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