As you already know, you can duplicate a ruby's spice (ruby's SECRET recipe) once you get it by placing it on the stove using any recipe and choosing just ruby's spice and pressing START. Then you will have another spice, ending up with two. You can get up to 99 and sell them to Van for ALOT of money!!! But what I'm going to tell you is how to GET a ruby's spice FOR SURE.

Befriend Ruby. Give her crops as often as you can. She seems to hate MY watermelons, B type, so don't give her those. Her above all favorite crop is potatoes. Once you've given her enough every day, you will know she is your friend because every time you walk by she will stare at you. She smiles, and when you give her crops(always, not just as her friend), she says "Really? I like you!" or something close to that. When she heads inside at night, follow her in to the kitchen. She should give you the RUBY'S SPICE.