It's post-humanity SF strangeness, set in a distant future where the Earth (?) is populated by living gems. Canonically they're genderless and potentially immortal, with cool weird gem biology, but in practice they:
(m) use masculine language while
(f) having female voice actors (some sounding boyish and some absolutely not), physiques that look feminine (flat-chested but with exaggerated hips and waists) and sometimes feminine behaviour (flocking around the male Kongou-sensei as if they're his personal harem, being modest in a feminine way about their own nudity, etc.) When the latter happens, incidentally, the discreet camera angles are treating the gems as feminine too.
I'll use female pronouns here, because that's what comes across on-screen. What you see and hear is female, but with occasional dialogue cues to remind you that technically they're not. This is a discussion point among English-speaking fandom, though, with support for male pronouns (following the original Japanese) and plural ones (which in practice just seems to make reviews slightly more confusing to read).
They "live" on an almost empty Earth. It's not sterile, but there's very little scenery or organic life. Lots of grass. It waves a lot. Everywhere you go, it's either grasslands or the ocean. There are also jellyfish, which are cleverer than you'd expect, but basically this is a future where evolution's taken some weird, sterile turns and now the dominant life-form is immortal and indestructible. Gems can break, mind you. Sometimes they do it to themselves through carelessness. Slip on a rock, fall down and whoops. Half my arm fell off! However they don't have blood and you'll suffer no permanent effect from, say, losing a leg. You'll still be able to scuttle around and fight on your remaining three limbs, like an big shiny insect. You'll also lose some memory. Gems' memories seem to be distributed throughout their bodies and stored in their "flesh", since they don't seem to have brains or internal organs. Decapitation can be fixed as easily as anyone else, although you'd want to avoid shattering. Even that's not fatal, but it'll be inconvenient if your friends can't find all the pieces.
Oh, and there are also (Hindu? Buddhist?) deities who regularly visit from the moon to kidnap gems. Their attacks are usually repelled, but if not then one of your friends is gone for good.
At first, the show seemed a bit sterile. Bland, empty setting. Gems who just get on with their straightforward lives and occasionally fight Lunarians. Our heroine, Phos, is a bit goofy and scatty, but that's not the same as being interesting. What got me interested was the extremely weird cool biology that ensues after she's swallowed by a giant acid snail. If you're intrigued by things that are unique, come here. They eat sunlight! They hibernate! Antarcticite is usually liquid and only crystallises in sub-zero temperatures! Besides, it's disconcerting to see gems regularly getting shattered, dismembered and/or head-split, only to shrug it off and continue their conversation.
There are philosophical discussions. There are gems whose body chemistry is toxic. There are a bunch of gems with personality quirks that don't help them get on with other people, which is unfortunate when there's almost nothing else left in the world.
It's a full-CGI show, but they've done some remarkable things with that. You couldn't do that translucence in traditional hand-drawn animation, for instance, although I should point out that the gems have white skin and they're only colourful and crystalline in their hair (or when injured).
It's quite a sober, dry show, but set in a fascinating SF world with the odd sight gag. Sometimes it's funny. It's got a lot of heart, but often filtered through stoic loners who aren't good at talking. It also has an ending that will immediately make you ask for Season 2, or perhaps copies of the manga. Incidentally it occurs to me that 2017 saw quite a few highly regarded anime set after humanity's demise. Girls' Last Trip, Kemono Friends, Land of the Lustrous... This show is delicate and a little distant, but fascinating.