Experiments and Studies of Luminous Stones

A luminous or phosphorescent stone, which has been named the Bologna stone, is the subject of a treatise published by the physician Mentzel in 1675. (Christiani Mentzelli, "Lapis Bononensis," Bilefeldiae, 1675.) The writer describes various experiments made to test the peculiar qualities of this mineral, which is partly a radiated or crystalline sulphate of barytes, and phosphoresces when calcined. It was sometimes called the "lunar stone" (lapis lunaris), because, like the moon, it gave out in the darkness the light it received from the sun. Mentzel also relates that the stone was first discovered, in 1604, by Vincenzio Casscioroli, an adept in alchemy, who believed that it would be a great aid in the transmutation of the baser metals into gold, on account of its solar quality. The place of its occurrence was Monte Paterno, near Bologna, where it appeared in the fissures of the mountain, after torrential rains.

The various phenomena of fluorescence and phosphorescence undoubtedly explain some at least of the legends regarding luminous stones, superstition or fantasy having here as in most other cases a certain substratum of fact. This class of physical phenomena has been made the subject of special investigation by the author, as many as 13,000 specimens of various minerals having been subjected to the most searching tests in order to determine their qualities in this respect. (See Kunz, "The Phosphorescence of the Diamond," Trans. N. Y. Academy of Sciences, 1890-91; Kunz and Baskerville, "The Action of Radium, Actinium, Roentgen rays, and Ultra Violet Light in Mineral and Gems,' Science, December 18, 1903). His interest in this field of research was greatly stimulated by a fortuitous happening. In 1891 his wife, while hanging up a gown in a closet one evening, saw that the diamond in a ring she was wearing gave off a faint streak of light which was very noticeable in the dark, and this fact led to a long series of experiments on the fluorescence, phosphorescence, and triboluminescence of the diamond. (See page 172) More than two centuries before, Robert Boyle made a similar set of experiments at night with a diamond which must have been an Indian stone, and which he describes as table cut, about one-third of an inch long and somewhat less in width; he remarks that it was a dull stone of very bad water, having a blemish with a whitish cloud covering nearly a third of the stone. (Boyle, "Works," London, 1744. The experiments were made October 27, 1663, and the results were communicated to the Royal Society the next day, the diamond which had been used being shown to the members at that time.)

The "Journal des Scavans" for 1739 gives certain tests of the luminous quality of diamonds made by Mons. Du Fay. In order successfully to observe this phenomenon, he prescribes that the experimenter shall remain in a darkened room for fifteen minutes, taking the additional precaution of closing one or both of his eyes. The diamond to be tested should be exposed to the sun's rays, or to strong daylight, for less than a minute, and when taken into darkness the luminosity, if observable, lasts twelve or thirteen minutes at longest. Not all diamonds show this quality, and nothing in their form or appearance serves to determine their possession of it. However, Mons. du Fay observed that the yellow diamonds, of which he tried a considerable number, were luminous. A single emerald, out of twenty that were tested, proved to be luminous. ("Journal des Seavans," 1739 of Amsterdam edition, citing "Hist. de l'Acad. Roy. des Sciences," 1735).

Boyle's experiments led to the discovery that some diamonds, when rubbed against wood or other hard substances, and even against cloth or silk, will emit a ray of light which seems to follow them; this is what is called triboluminescence.

The power of absorbing sunlight or artificial light and then giving it off in the dark is only possessed by certain diamonds. These are Brazilian stones, slightly milky in tint, or blue-white as they are often termed, and it is an included substance and not the diamond itself that possesses the power of storing up light and then giving it out. Willemite, kunzite, sphalerite (sulphide of zinc) and some other minerals possess the same power. Their peculiar property may be due to the presence of a slight quantity of manganese or to that of some of the uranium salts. That it is only the ultra-violet rays that are thus absorbed by these diamonds is proved by the fact that the phenomenon is not observable when a thin plate of glass is interposed between the sunlight or artificial light and the diamond, as glass is not traversed by these rays. The still undetermined substance to whose presence in diamonds of this type the special class of phenomena must be due, was named by the author tiffanyite, in honor of the late Charles L. Tiffany (1812-1902), founder of the firm of Tiffany & Company. (See Transactions of the New York Academy of Sciences; 1895.)

On the other hand all diamonds phosphoresce when exposed to the rays of radium, polonium, or actinium, even when glass is interposed. Treating of some of the aspects of phosphorescence in diamonds, Sir William Crookes says ("Diamonds," a lecture delivered before the British Association at Kimberley, Sept. 5, 1905; London, 1905. See also the same author's "Diamonds," London and New York, 1909.):

"In a vacuum, exposed to a high-tension current of electricity, diamonds phosphoresce of different colours, most South African diamonds shining with a bluish light. Diamonds from other localities emit bright blue, apricot, pale blue, red, yellowish-green, orange, and pale green light. The most phosphorescent diamonds are those which are fluorescent in the sun. One beautiful green diamond in my collection, when phosphorescing in a good vacuum, gives almost as much light as a candle, and you can easily read by its rays. But the time has hardly come when diamonds can be used as domestic illuminants!

By permission of Mrs. Kunz, wife of the well-known New York mineralogist, I will show you perhaps the most remarkable of all phosphorescing diamonds. This prodigy diamond will phosphoresce in the dark for some minutes after being exposed to a small pocket electric light, and if rubbed on a piece of cloth a long streak of phosphorescence appears."

The luminescence produced by heat is wonderfully marked in the case of chlorophane, a variety of fluorite. A Siberian specimen of a pale violet color emitted a white light merely from the heat of the hand; boiling water caused it to give out a green light, which was so greatly intensified when the specimen rested on a live coal that the radiance could be discerned from a considerable distance. Similar phenomena were observable in the case of chlorophane from Amelia Court House, Va., and the writer found that specimens from this source also exhibited strong triboluminescence, resulting either from contact with one another, or with any hard substance. (Kunz, "Gems and Precious Stones of North America," New York, 1890.)

As the terms fluorescence and phosphorescence are sometimes rather carelessly employed, it may be well to note here that while both terms are used to denote the luminescence of a non-luminous body resulting from the action of light rays, of the electric current, or of radiant energy of any kind, as well as from heat, fluorescence signifies a luminosity which only continues so long as the exciting cause is present, while phosphorescence means a luminosity persisting for a longer or shorter period after the exciting cause has ceased to operate directly. The latter term therefore denotes a luminous energy stored up in the formerly non-luminous body and emitted by it for a certain time, at the expiration of which it again becomes non-luminous. Other special designations of induced luminosity in minerals are triboluminescence, the emission of light as a result of friction and thermo-luminescence, a term used to denote light-emission excited by moderate heating, even by the warmth of the hand.

An old treatise in Greek, said in its title to come from "the sanctuary of the temple," and containing material, partly of Egyptian origin, may help us to understand something of the processes employed by a temple priest to impress the common people by the sight of luminous gems. The writer of the treatise declares that for the production of "the carbuncle that shines in the night" use was made of certain parts (he says "the bile") of marine animals whose entrails, scales and bones exhibited the phenomenon of phosphorescence. If properly treated, precious stones (preferably carbuncles) would glow so brightly at night "that anyone owning such a stone could read or write by its light as well as he could by daylight. (Collection des anciens alchemistes grecs," ed. by M. Berthelot, trans., Paris, 1887, 1888.)

In the Annales de Chimie et Physique, the great French chemist, M. Berthelot, discusses this matter and expresses the following opinion (Sur un procede antique pour rendre les pierres precieuses et les vitrifications phosphorescentes," Annales de Chimie et Physique, 6th ser.):

"The texts leave no room for doubt as to the employment by the ancients of precious stones rendered phosphorescent in the dark by the superficial application of tinctures composed of materials whose phosphorescent quality is known to us. Although this luminescence, due to an application of organic oxidizable materials, could not well be durable, still it might be made to last several hours, perhaps several days, and it could always be renewed by repeating the application."

The use of jewelled ornaments to heighten by their luminosity in obscurity or in darkness the effect produced by a sacred image, and to stimulate religious awe in the beholder, is testified to by the ultra-Protestant traveller, Fynes Moryson, Gent., who went to Italy in 1594. Of his visit to the Santa Casa in Loreto, he says that he himself and two Dutchmen, his companions, were permitted to enter the inner chapel of the sanctuary, "where," he proceeds, "we did see the Virgin's picture, adorned with pretious Jewels, and the place (to increase religious horror) being darke, yet the Jewels shined by the light of wax candles." Although there is no question here of naturally luminous gems, this might have been the impression produced upon a more sympathetic pilgrim. (Moryson, "An Itinerary containing his Ten Yeeres Travell through the Twelve Dominions," etc., Glasgow, 1907-8.)