National Geographic : 1951 Nov
© National Geographlic Society +" Daylight Fails to Release Rocks' Flashing Spectral Hues These specimens of willemite, a zinc silicate, came from Franklin, New Jersey, only place in the world where the mineral is found in sufficient quantities for zinc smelting. White spots in the smaller rock are calcite, principal ingredient of limestone and marble. Ektachromes )byWillard It. Culver + Drab Stones Change to Brilliant Jewels in Ultraviolet Light Willemite becomes green, calcite red. Blue spots are caused by imperfect filtering of ultraviolet rays. Energy from these rays makes atoms vibrate, emitting colored light. Fluorescence aids in mineral identification by disclosing characteristic hues.