Curium

It should not be surprising that curium is radioactive as it is named after Marie Curie. The radioactivity of this material is what is unexpected. Curium glows purple when the lights are turned off. If you try to immerse it, water will begin to boil. It emits radiation in the forms of alpha, beta, and gamma, making it a serious concern to have around. Because of this, it is infrequently intentionally created, and when it is, it is done under controlled laboratory settings.

Curium can be created in a nuclear reactor by hitting plutonium with uranium. Therefore, it's not a mistake you're likely to commit. There are roughly 20 different curium isotopes, some of which have millions of years for their half-lives. It is actually the most radioactive element that science has so far been able to isolate.