I really do like Houseki no Kuni's wonderful characters and whimsical fantasy tone -- until it loses it along with most of its charm and Phosphophyllite's sanity. From chapter 19 onward, Houseki no Kuni's tone and pacing suffer bouts of schizophrenia. Excessive and often repetitive action scenes drag out the midsection of the currently released chapters, littered with gratuitous "deaths". The most interesting and nuanced characters are sidelined until all that's really left to focus on is Phos, Cairngorm, and Adamant -- and maybe Cinnabar, too, since the mangaka really likes to remind you that Cinnabar exists just as you're about to forget.
Houseki no Kuni's greatest sin is wasting your time, which comes to a head when Phos is forced to learn about the existence of humans again despite already learning the information some 40 chapters earlier. After 40 months (this is a monthly series, so storytelling time is precious), Phos finally learned information they had already learned and then forgotten. No new information is gained, so for the reader this is a colossal waste of storytelling time. The long stretch of fights in the midsection of the 61 chapters play like a groundhog day loop of Phos's failures, wearing down your energy to continue reading.
The art is simplistic and rough, with Clamp-like spaghetti limbs. Each of the gems wear the same clothes and have roughly the same face, making them hard to distinguish save for their distinct hairstyles. The roughness of the art begins to take its toll the more Phos's design changes with each "upgrade" they receive. These upgrades come in the form of new materials to replace Phos's lost body parts, roughly corresponding to the Seven Treasures of Buddhism and meaning to date there are likely 2 more upgrades left. This gives Phos a Frankenstein's monster appearance of mixed colours and a messy design that doesn't do the art any favours.
I really do want to like Houseki no Kuni more. A lot more. But I'll keep reading it anyway.