Bob Weber February 01, 2021 - 1:00 AM EDMONTON - Sort of silvery in its raw state, sort of metallic but not really, selenium embodies contradiction. It's fairly rare â the 68th most common element â but often found with coal. Tiny amounts are essential to health â selenium helps the body fight tissue-damaging free radicals â but too much is harmful. Scientists say selenium was brought to the Earth's surface by explosive volcano eruptions in the Cretaceous Era, which ended 66 million years ago. It was absorbed and concentrated in the shells of sea creatures, which is why it's relatively common in carbon-rich deposits from that era.