What causes a pokemon to evolve with an evolution stone I mean like do they have to be touching it for a certain amount of time or something? Say a Pikachu doesn’t want to evolve and comes into contact with a Thunder stone, would it still evolve?

Evolution is a bit of a complicated subject. For the most part, you can’t actually force a pokémon to evolve, even if all conditions are otherwise met. The pokémon must also wish to evolve in order to initiate the process.

Before I say anything more, allow me to clarify that evolution stones contain a form of radiation or energy that can trigger pokémon evolution by physical contact. However, evolution is ultimately a matter of will. A pokémon that does not wish to evolve can actually touch an evolution stone without consequence if they will themselves to remain in one form. Likewise, other pokémon may evolve immediately if they wish to by letting their internal guards down, so to speak.

However, it should be noted that resisting evolution for long periods of time may be stressful to a pokémon, so if yours is clearly attempting to resist, it’s highly recommended that one removes the evolution stone as soon as possible and, if necessary, apply an everstone to absorb the evolutionary energy.