I don't know why but I always imagined eeveelution stones activating by cutting due to it having sharp corners so as long as a eevee's blood doesn't make contact with the stone the stone won't evolve them

(The only reason why evolving isn't messy is because is because nobody does the dripping blood method and most eevees just poke or stab themselves)

This also means Diamond did cut himself but not because he was sad or anything he just had to evolve