What is the appeal of this show, why is it rated so highly?

Unique premise that's well executed with unique characters that aren't one dimensional, (they have goals aspirations and worries).

Looks like some ugly CGI

Excellent aesthetic quality, texture quality, lighting quality, expressive characters in both body language and facial expression.

story didn't really seem to exist?

-Introduction of premise of personification of gems.
-Concept of memories being held in fragments.
-Concept of fragility and hardness values.
-Introduction of major characters, (Phosphophyllite, Kongou, Cinnabar) as well as several minor characters.
-Phosphophyllite's motivation: join everyone in combat.
-Phosphophyllite's responsibility: write an encyclopedia.
-Phosphophyllite's second motivation: Cinnabar.
-Cinnabar's goal: find a place she belongs, (on the moon or otherwise)

How much more plot do you want in an opening episode?