Ruby's Spice is an ingredient that can be used in Curry, Hot Curry, & Mushroom Curry. You only get one per game. After befriending Ruby, enter her kitchen when she's there. She will ask if you are hungry and if you've been cooking at home. If you say "yes", she will give you the spice. It will never run out when you use it, but can be stolen by Murrey or sold, and once it's gone, it's gone forever.
However, there is a known glitch to clone it and can be taken advantage of to earn cash by selling the extra ones to Van, but make sure you keep at least one of them.
If you cook the Ruby Spice by itself (ex. Salad > Ruby Spice > Select > A) then press the B button, you should have two of them. You can keep doing this as many times as you like.