A 43-carat albite from Burma (at left), 76-carat tourmaline from Brazil, and 30-carat wernerite from Burma exhibit a strong cat’s-eye effect because of reflection from inclusions in parallel arrangement within the stones. (Actual size.)
Asterism (star effect) is caused by parallel inclusions arranged in several directions related to the crystal structure of the gemstone. Two rays in the 175-carat, 6-rayed star garnet from Idaho (at left in photo) are weaker than the other four because of fewer inclusions in that direction. The 23-carat star orthoclase from Ceylon shows brightly all of its four possible rays. (Actual size.)
The effect of a gem on light may be more than the production of color. Several of the so-called phenomenal stones are prized for other effects. Holes, bubbles, and foreign particles, when properly aligned in parallel groupings, can produce interesting light effects. The play of colors of opal and labradorite, the chatoyancy or silky sheen of tiger’s-eye and cat’s-eye, the opalescence or pearly reflections of opal and moonstone, and the asterism or star effect of rubies and sapphires are caused by the reaction of light to minute inclusions or imperfections in the gemstone.
When light passes into or through a gemstone with little or no interruption, the stone is said to be transparent, as opposed to a stone through which light passes with greater difficulty, and which is said to be either translucent or opaque, depending on the degree of light interruption.
Rays of light passing into a gemstone are refracted (bent) in varying amounts depending on the gem species and also on the angle at which the light strikes the stone. The light rays are reflected back toward the top of the stone by internal faces (facets), and they are refracted again as they leave.
How a gem refractometer, a simple device to operate, is used to measure quickly the refractive index of a cut gemstone. A light beam passing through the opening (A) is reflected from the table of a gemstone (G) through a lens system (L) and, by prism (P), into the eye of the observer (E). The maximum angle of reflection (N), which depends on the refractive index of the gemstone, controls the angle at which the beam comes through the eyepiece (EP). The refractive index is read directly from a scale in the eyepiece.
The action of a gemstone upon the light which strikes its surface and is either reflected or passed through it sometimes results in highly desirable effects that enhance its beauty and aid in its identification. Light passing into a stone is bent from its path, and the amount of bending (refraction) depends upon the species of the gemstone. When the degree of bending can be measured, the gem species can be identified, since very few species of gemstones bend light to exactly the same degree. An instrument called a gem refractometer is used to determine the degree to which cut stones refract, or bend, light. The measurement obtained is the refractive index of the gemstone.