The thunderstone is a evolution-ing or evolv-ifying substance. It causes some thunder type pokemon to evolve.
The thunderstone is a mutagen and it's effects are mutagenic.
an agent (as a chemical or various radiations) that tends to increase the frequency or extent of mutation.
"Mutagen." Merriam-Webster.com. Merriam-Webster, n.d. Web. 11 Sept. 2016.
: inducing or capable of inducing genetic mutation
"Mutagenic." Merriam-Webster.com. Merriam-Webster, n.d. Web. 11 Sept. 2016.
It's an evolution inducing substance. It induces the evolution.
induce to cause something to happen -Cambridge Dictionary