Folktales about luminous gemstones are an almost worldwide motif in mythology and history among Asian, European, African, and American cultures. Some stories about light-emitting gems may have been based on luminescent and phosphorescent minerals such as diamonds.
Mineralogical luminosity
First, it will be useful to introduce some mineralogical terminology for gemstones that can glow when exposed to light, friction, or heat. Note that the following discussion will omit modern techniques such as X-rays and ultraviolet light that are too recent to have influenced folklore about luminous gems. Luminescence is spontaneous emission of light by a substance not resulting from heat, as distinguished from incandescence, which is light emitted by a substance as a result of heating. Luminescence is caused by the absorption of energy that is released in small amounts. When the energy comes from light or other electromagnetic radiation, it is referred to as photoluminescence; which is divisible between fluorescence when the glow ceases immediately with the excitation and phosphorescence when the glow continues beyond the period of excitation. Two types of luminescent phenomena are relevant to crystalline materials. Triboluminescence generates light through the breaking of chemical bonds in a material when it is rubbed, pulled apart, scratched, or crushed. Thermoluminescence re-emits previously absorbed electromagnetic radiation upon being heated (e.g., thermoluminescence dating). The American geologist Sydney Hobart Ball, who wrote an article on "Luminous Gems, Mythical and Real", outlined the history of discoveries about luminescent and phosphorescent minerals. Most diamonds are triboluminescent if rubbed with a cloth, and a few are photoluminescent after exposure to direct sunlight. Both diamonds and white topaz may phosphoresce if heated below red heat. The phosphorescent quality of diamonds when heated by sunlight is usually believed to have been first revealed by Albertus Magnus (c. 1193–1280) and it was apparently rediscovered by Robert Boyle in 1663, who also found that some diamonds will luminesce under pressure. According to Prafulla Chandra Ray, the Indian king Bhoja (r. 1010–1055) knew that diamonds can phosphoresce. The luminescent Bologna Stone (impure barite), which was discovered by Vincenzo Cascariolo in 1602, was sometimes called "lapis lunaris" ("lunar stone"), because, like the moon, it gave out in the darkness the light it received from the sun. In 1735, the French chemist Charles François de Cisternay du Fay determined that lapis lazuli, emerald, and aquamarine were luminescent. Josiah Wedgwood, in 1792, found phosphoresce from rubbing together two pieces of quartz or of agate, and wrote that the ruby gives "a beautiful red light of short continuance." Edmond Becquerel reported in 1861 that ruby fluoresces better than sapphire, red feldspar fluoresces, and crushed orthoclase will flame. In 1833, David Brewster discovered the fluorescence of the mineral fluorite or fluorspar. However, the English naturalist Philip Skippon (1641–1691) stated that one Monsieur Lort, of Montpellier, France, a "counterfeiter" of "amethysts, topazes, emeralds, and sapphires" found that on heating "fluor smaragdi" (Latin for "flowing emerald/beryl/jasper") in a pan of coals and afterwards "putting it in a dark place (it) shines very much: At the same time several other stones were tried but did not shine" (1732 6: 718). Some fluorite, particularly the variety chlorophane (aka pyroemerald and cobra stone), may become very faintly luminescent simply from the heat of one's hand. Chlorophane is unusual for combining the properties of thermoluminescence, triboluminescence, phosphorescence, and fluorescence; it will emit visible spectrum light when rubbed, or exposed to light or heat, and can continue emitting for a long period of time. Among the gravels of the Irtysh River, near Krasnoyarsk, Russia, the German mineralogist Gustav Rose recorded seeing chlorophane pebbles that shone with brilliancy all night long, merely from exposure to the sun's heat. For luminous gem myths, Ball concludes that while it is "not impossible that the inventors of certain of the [luminous gem] tales may have been acquainted with the luminosity of gems, in my opinion many of the tales must be of other origin". Scholars have proposed many identifications for myths about luminous gemstones described for over two thousand years. Most frequently rubies or carbuncles (often red garnets), which classical and medieval mineralogists did not differentiate, and less commonly other gems, including diamonds, emeralds, jade, and pearls. The American sinologist Edward H. Schafer proposes that the phosphorescent "emeralds" of classical antiquity, such as the brilliantly shining green eyes of the marble lion on the tomb of King Hermias of Atarneus (d. 341 BCE) on Cyprus, were fluorite, even though the Hellenistic alchemists had methods, "seemingly magical, of making night-shining gems by the application of phosphorescent paints to stones", the most famous being their "emeralds" and "carbuncles". The names of some luminescent gemstones etymologically derive from "glow" or "fire" words (e.g., pyroemerald for "chlorophane" above). The OED defines pyrope (from Greek Πυρωπός, lit. "fire-eyed")" as: "In early use applied vaguely to a red or fiery gem, as ruby or carbuncle; (mineralogy) the Bohemian garnet or fire-garnet"; and carbuncle or carbuncle-stone (from Latin "carbunculus", "small glowing ember") as: "A name variously applied to precious stones of a red or fiery colour; the carbuncles of the ancients (of which Pliny describes twelve varieties) were probably sapphires, spinels or rubies, and garnets; in the Middle Ages and later, besides being a name for the ruby, the term was esp. applied to a mythical gem said to emit a light in the dark".
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