Yeah I mean the game is intentionally confusing.

You have these things called eternal crests which cost 160 “gems” (the real money currency). These let you run a special dungeon that guarantees a drop of a legendary gem rated between 1 and 5 stars. In fact if you fail the dungeon somehow it’ll refund the crest and let you try again.

Apparently the drop rate for the 5 star gems is 4.5%, so my napkin math works out to around $50 per 5 star legendary gem.

I can’t go back and check my math because I uninstalled the game.