Brief Review:

Stunning animation using mixed media, mainly CGI, and doing it with great definition and color contrast and movement to make something entirely pleasant to look at. Besides the impressive animation lay an incredibly unique fantasy world disconnected from all of humanity as we know it.
Houseki No Kuni takes place on an Earth composed of just a single island, inhabited entirely by sentient gems (in the anime anyway) who struggle in their never-ending fight against the creatures of the moon. This anime shows a coming-of-age story portrayed in such an understandable yet so distant from the normal way. It's through the point of view of our main character, Phos who slowly loses themselves and eventually not even knowing if they are themselves, going on to have even more identity crisis in the manga. It shows a descent into the deepest pits of our greed and the burdens of growing up too quick for your good.
The soundtrack helps highlight the feelings of the current mood of the show with the score invoking a certain emotion within you, without even having to be watching the scene of the show it matches with.
Overall, its soundtrack complements every other aspect of the show, the mood, the art, and of course the plot. It's an amazing work and completely worth checking out as well as the manga which continues the story at the same, possibly greater storytelling quality. The manga continues from the ending of the anime and tells an even darker story. One of a descent into madness rather than a coming of age story.

A Deeper Look:

This is my view of the deeper plots and elements of the writing of Houseki No Kuni. Major spoiler warning for the entire anime and some of the manga. Phos goes through a lot of heavy issues and self-realizations throughout the season. Viewing the death of their friends, loss of self, betrayal, and realizations about the truth of this world. Phos learns to question even their Sensei who had taken care of them since they were formed. They learn to be on their guard even from seemingly friendly creatures. Phos even forgets a majority of their memories by the end of the season, having lost body parts with new memories from other vessels to deal with. This all culminates into having a strained relationship with everyone Phos knew in the best, truly being an entirely new person by the end of the show, a new Phos in a way. Phos perfectly encompasses the loss of self and serious mental issues. Phos is just one of the many characters in Houseki No Kuni each character has their own troubles they go through with little to no coping mechanisms, these gems are then forced to fight constantly going through even more pain. To some people this fate might be even worse than death.