I first want to state that by the time of writing this review, I still haven't read the manga, even though I have plans to. So, my observations and rankings are solely based on the anime, which felt more like a prologue and sadly won't be getting a second season (at least any time soon).

Worldbuilding

I want to get straight into this anime's forte, the world. When writing a fantasy world, it is important to have the fantasy rules be rigid yet at the same time also flexible. The rules must be rigid in order to feel like a show that has successfully carved its own reality with its own rules, and failing that can make the show feel less grounded and really gimmicky. However, it must also have some flexibility in those rules so that it isn't too limited by its own formula, and can have enough space to try out new ideas, surpass expectations and implement plot twists that don't feel unnatural. This is a hard middle ground to catch, but Houseki no Kuni excels at this.

The premise (and by extension the world) is quite basic but really imaginative and sticks to its own rules. Mysterious lifeforms known as "gems" inhabit a small and lonely island which is under constant attack by weird people from the moon called Lunarians, people who want to abduct these gems and use them for their own desires. This is the vague yet intriguing world revealed with the starting episode, and as the show continues more plot points are weaved into the show. I want to keep this review spoiler free, but many interesting things are revealed about both the history of the world, the Lunarians, the gems and other lifeforms as the show continues. This is done in a way that doesn't feel out of place at all for the show and its original premise, but also expands the horizons of what this show can be, and a sense of mystery is always masterfully preserved. The world building is an easy 9/10, but depending on how the manga progresses it can be a 10/10 in a later season (if we ever get any).

Characters

The characters of Houseki no Kuni are pretty well done too, but I won't be analyzing them too much since not a lot is shown to us in this first cour. It really seems like this show needed a second cour (or a second season), especially when it comes to character development. Our main character, Phos, is a bratty annoying kid that does experience character development about two thirds into the show, but by then it feels like there isn't enough time to fully utilize that development, and it does seem a bit too sudden. Our other main/important characters like Kongo, Shinsha, King, Diamond and Bort seem really interesting but none of them have much focus or development for now, so it is hard to analyze them deeply. I love Bort's harsh attitude and sense of responsibility as it is executed very well, Kongo's aura of mystery and kind nature makes him an extremely intriguing character and Shinsha's tsundere nature screams that there is something up with her past but none of them get enough time to get fleshed out.

The side and supporting characters are... alright at best. They rely too much on quirky and one dimensional personalities like Red Beryl's love of dresses, Morganite's snarky attitude or the oblivious kuudere Amethyst sisters. Even the characters that have some depth like Rutile or Alexandrite rely too much on their quirks (mad scientist and obsessiveness over Lunarians respectively), even if those quirks are the result of something deeper about their past. It just isn't reflected very well and overall is a poor portrayal of these characters. I'd give the characters a 7/10 for now, but further seasons can drive that up to an 8 or even a 9.

Art and Music

The art of this show needs no introduction really. Houseki no Kuni is famous for its amazing implementation of CGI. The animation of facial features and expressions looks just as natural as any other anime, fighting scenes cleverly utilize the 3D models, and landscapes look absolutely stunning. Even though I have some subjective dislikes about the art style at times, I'd say objectively the art is a 9/10. I especially loved how the Lunarians were drawn, they look like paintings and reliefs from ancient civilizations like Assyria or Egypt.

The soundtrack was also great. Some of the tracks are simply a joy to listen to and others are almost trancelike. The music fits the art style and the mood of their respective scenes very well. Is it the next Madoka Magica? No, but it's pretty damn good. 9/10.

The opening is gorgeous visually and the music is wonderful as well. It can sound a bit generic after the 50 second mark but it still stunningly beautiful imo, especially for the first 50 seconds. 9/10. The ending is also an artistic masterpiece but I didn't like it very much musically, it felt way too disorganized. 6/10.

Overall, I'll give this section a 9/10 overall.

Conclusion

Houseki no Kuni is a great show with superb worldbuilding, gorgeous art and music, and great characters. Its single fault in the end is only being 12 episodes long. This cour feels like it is a prologue for a much greater story. You could chalk that up to poor pacing, but I think the pacing was mostly fine. There just wasn't enough time for the world and especially the characters to truly flesh out. All we can do now is hope for a S2 after the final Beastars season, but that's just hopium.