Ruby (Stick) CE

their only use is EXP for your CEs afaik
treat them like ember cards for your CE

They're used to level your Craft Essences, which is easy for low-leveled ones, and super difficult for high-leveled ones.

One of the better ways to use them is to set up XP Bombs. It's a really QP-efficient way to boost your high-leveled CEs. You can read more about it here:
https://grandorder.wiki/CE_Bomb_Guide