Sapphire, a member of the corundum family of gems, exhibits a broad color range. Impurities in clear corundum tint the naturally colorless stone yellow, green, pink, purple or blue. Purple sapphires share their royal hue with another vivid purple gemstone, the amethyst. Although their color ranges overlap, the two gem types have little else in common.
Corundum rates a 9 out of a possible 10 on the Mohs hardness scale; only diamond is harder. Quartz varieties, including amethyst, fall much lower on the Mohs scale at 7. Both purple sapphire and amethyst will scratch relatively softer window glass — a 6 on the hardness scale — but other forms of quartz will mar amethysts while leaving sapphires untouched. Older amethyst jewelry shows the evidence of this softness in the edges of its facets, which wear visibly within a few decades of daily wear.
Purple sapphires and amethysts have distinctive chemical structures. Aluminum oxide (Al2O3) describes all colors of corundum; purple varieties contain vanadium or chromium atoms in place of a small number of aluminum atoms within its crystalline structure. Quartz varieties consist of silicon and oxygen as silicon dioxide or silica (SiO2). Amethyst derives its violet color from natural irradiation of iron impurities within the mineral. Both the crystalline structure of amethyst and the elements that comprise it differ from those of sapphire.
Although both stones’ color ranges resemble one another, their brilliance does not. Sapphire has a higher refractive index of 1.762 to 1.778. Amethyst’s refractive index of 1.544 to 1.553 means that the stone has less sparkle than a similarly colored sapphire. Gemologists measure refractive index as one distinguishing characteristic of a gem. Some gems’ refractive indices overlap, but sapphires and amethysts exhibit such a marked difference in their brilliance that comparing samples of each stone will reveal which is which even to the unaided eye.
The mineral quartz in its various forms makes up a significant portion of the earth’s crust. While not all quartz is gem-quality amethyst, the clear purple stone is more common than any form of corundum, including the purple variety. Large deposits of gem-quality amethyst occur throughout North America, South America, central Europe and much of Asia. Sapphires occur in fewer regions. Jewelers price the stones accordingly; semi-precious amethyst gems fetch a lower price than purple sapphires of lesser quality and size.
References and ResourcesInternational Colored Gemstone Association: Amethyst
The Mineral and Gemstone Kingdom; The Precious Gemstone Sapphire; Hershel Friedman
Ruby-Sapphire; Walking the Line in Ruby and Sapphire; Richard W. Hughes; updated Jan. 2011
Gemological Institute of America; Sapphires Reportedly from Batakundi/Basil Area; Vincent Pardieu, Kamolwan Thirangoon, et al; Apr. 2009
Gem Select; Refractive Index Chart; Dec. 2007