The Beauty of Phenomenal Stones

With the crystal-clear beauty of the chemically produced colors, we have still not reached the end of the chain of wonders, so incredible to the layman. However bewildering the almost inexhaustible variety of colors, yet more bewildering are the twinkling light displays of the phenomenal stones—of cat's-eyes and star stones, of moonstone, opal, and spectrolite. They exert less of a general attraction but an all the more distinctive beauty appealing to personal taste. Gemstone lore and superstition could scarcely find yellow diamond rings anywhere more fruitful soil than in these intangible visions reflected from the unplumbable depths of the stones' interiors or apparently sweeping over their surfaces. For the uninitiated, these visions are wreathed with inexplicable mysteries, but in actual fact they originate from purely optical properties of light rendered visible by structural peculiarities of the gemstones concerned.

Chatoyancy, or the cat's-eye effect, results from reflections of light from microscopically fine fibers or hollow channels, lying parallel to one another and traversing the entire body of the stone. If a domed surface curves above them, the light shrinks into a narrow line. Well-known examples are the golden-brown tiger's-eye, wedding bands gold the blue falcon's-eye, and especially the most valuable of all, the bamboo-green to honey-colored chrysoberyl cat's-eye. Asterism, or star formation, is in fact merely a multiplication of the single chatoyancy, in which two or three rays of light cross one another and thus make up four- or six-rayed stars. The cause lies in the two or three systems of extremely fine, parallel fibers or minute needles which intersect at 90 degrees or 120 degrees. Star rubies and star sapphires exhibit this phenomenon at its most effective with six-rayed stars. There are, however, star spinels and star garnets too, displaying four- and six-rayed stars alternately.

Adularescence is the name given to the pale blue, wedding bands surging shimmer of light which glides over the convex surface of a moonstone. It originates from interference of light when it strikes the fine lamellar structure of this gemstone. Interference is likewise responsible for the blue gleam of common labradorite.

Diffraction of light by the gratinglike packing of the spheres of which opal is composed causes its exquisite, color-flecked play of flames. The phenomenon is comparable with the colored image of a street lamp seen through the weave of an open umbrella.