Selenium in Sandstone

Selenium is in the same chemical family as oxygen, sulfur, telurium, and polonium: oxygen and sulfur are nonmetals, polonium is a metal, and selenium and tellurium are semiconductors (i.e., their electrical properties are between those of a metal and an insulator).  Nevertheless, tellurium, as well as selenium, is often referred to as a metal when in elemental form.  In the late 1990's, the use of selenium (usually with bismuth) as an additive to plumbing brasses to meet no-lead environmental standards became important. Image credit: By James St. John - Selenium in sandstone (Westwater Canyon Member, Morrison Formation, Upper Jurassic; Section 23 Mine, north of Grants, New Mexico, USA) 1, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=40932329