Not sure what kind of rule you're looking for, but English orthographic c is 'hard' (i.e., a stop rather than a sibilant or fricative) before a, o, or u (mainly back vowels). It is 'soft' before e or i (or y) (mainly front vowels).

So, cat, caught, cot, coat, couch, and coot, but cedar, cilia, or cyan. But, ketamine, kit, or kyne. I'm sure some of the other folks will come up with some interesting exceptions.

Ceci n'est pas un seing.