gems, beryl, emerald, causes of color

Williams investigates the cause of the green color of emerald through chemical examination of emeralds and related beryls. He reviews earlier explanations of emerald coloration and analyzes specimens to determine the nature and quantity of the coloring constituent. Particular attention is given to chromium, whose presence he identifies as responsible for the characteristic green color of emerald, countering earlier claims that organic matter might be the principal coloring agent. Williams compares emerald with differently colored and colorless beryls and discusses the relationship between small quantities of chromic oxide and color intensity. The paper is an important nineteenth-century contribution to the chemistry of gemstones and to establishing chromium as the principal chromophore of emerald.