Selenium is a naturally occurring element with a periodic number of 34. It was discovered in 1817 and is found impurely in metal sulfide ores.
Selenium has an interesting property that most elements do not have called photoconductivity. Photoconductivity is a phenomenon where a substance actually becomes more conductive when visible light is shown on it!
Not only does Selenium become more conductive when light is shown on it, but it is actually an insulator when there is not light on it!
Selenium is mostly used in glass production, with over 50% of its consumption used in giving glass a red color after the selenium is added.
For all the Doctor Who fans out there, I cannot help but wonder if the "Weeping Angels," a reoccurring foe of the doctor that can only move when they are not seen, aren't made of some other element that has an anti-photoconductivity phenomenon!