1 Rainbow Gem will be received upon first clear of each cavern
Scissor Cavern, Entrance is a regular Friday Limited Event, meaning it appears for 24h each Friday.
Scissors Cavern, Inner Grotto was not a regular Friday event, it used to appear only on some special Fridays (also for 24h). Around late 2016/early 2017 it became available on each Friday. It has no rainbow boss, just two chances of catching regular plated lobsters (there is no rainbow plated lobster in the game - yet).