The Periodic Spiral assigns the color indigo (a deep blue) to sulfur, hafnium, osmium, mercury, and polonium. The last three of these elements are poisonous, and both sulfur and osmium have pungent odors. Sulfur burns in oxygen with a deep blue flame. The rich indigo blue color of lapis lazuli is due to the sulfur that is inherent in the structure of lazurite. The color of hafnium metal is sometimes described as brilliant blue, while osmium metal is variously described as blue-gray, blue-black, or blue-black, depending on its form. A few curies (1 curie = 3.7 x 1010 Bq) of polonium exhibit a blue glow, caused by excitation of the surrounding gas. Mercury vapor emits blue light. "Blue mass," a concoction of mercury and chalk, was a commonly prescribed medication in the mid-nineteenth century.