Chatoyancy: Popularly known as cat’s eye gem, (chat in French means cat). Chatoyancy refers to a phenomenon akin to opening and shutting of cat’s eye. These gems have a single sharp band (sometimes two or three bands) running across the domed cabochon surface. The Chrysoberyl cat’s eye gem is very specifically cut as cabochon to highlight chatoyancy. The rutile silk is aligned perpendicular to the crystal. So when light falls on it, the sharp band can be seen. In the best stones, the chatoyant Chrysoberyl cat’s eye visually separates the surface into two halves creating milk and honey effect as the stone is moved under light. Other gems that can show cat’s eye phenomenon are Tourmaline, Moonstone as well as Quartz.

Adularescence is named after “adularia”, (Historical name for Moonstone). The effect is due to light interference caused by layered atomic structures within the stone. The gem displays a billowy floating light which appear to come from below the surface. The term Shiller or Schiller is sometimes used to describe this effect. As in other optical phenomenon, the size and distance from layer to layer influences the colors we see. The colors can be white, blue or rainbow hues. Adularescence appears in other gemstones as well notably, Opal, Rose Quartz and Chalcedony.

Aventurescence owes its beauty to structural features that diffract or scatter light; aventurescence is a consequence of reflection. When disk or plate like inclusions of another mineral are present which are highly reflective, they act as tiny mirrors. The gem sparkles and glitters. This glitter effect is called aventurescence. The term Shiller is also used for this glowing effect. The most common reflectors are copper, hematite and mica. The term Aventurescence is derived from the word “a ventura” meaning “by chance”. This effect is seen in Aventurine Quartz, Sunstone Feldspar and a man-made stone called Gold Stone.

Iridescence is derived from the Greek word “iris” meaning rainbow. It is caused by diffraction of white light as it is passing through small openings such as pores and slits or when light passes through thin layers of different minerals with different refractory indices. A prism effect causes light to separate into various spectral colors. Iridescence is the most common optical phenomenon and the most mesmerizing one. It can be easily appreciated in nature such as on the neck of the pigeon, soap bubble, wings of a butterfly or iridescent beetle. Gems with remarkable iridescence are thinly sliced Iris Agate, Fire Agate and Ammolite. Coating the surface of an ordinary gem with a chemical and heating can produce man-made iridescence.

Play of Color: Iridescence in precious opal is called “Play of Color”. Opals are generally divided into precious and common, based on whether they have Play of Color or not respectively. Play of Color is seen as shifting patches of spectral colors on a gem’s surface and/or in its interior. This phenomenon is actually caused by a unique ultra structure of the Opal. The Opal is made of spheres of silica, which are arranged in closely stacked layers and have air or rarely liquid between them. These spaces with air or liquid cause diffraction and split light into various colors. Thus we get a shifting group of colors that flicker and move as we move the stone. The colors we see depend on the size and regularity of arrangements of the spheres. With larger spheres more red (more value) and with smaller spheres more blue is seen (less value). As with any gemstone, color preferences are subject to individual tastes.

Color Change: Some gemstones appear very different in daylight (natural light) and incandescent light. This phenomenon of “color change” is typically observed in a gem called Alexandrite and this effect is also called “Alexandrite x Effect”. The Alexandrite appears green in daylight and reddish purple in incandescent light. Other gems that display color change phenomenon occasionally are Sapphire, Spinel, Garnet and Tourmaline.

References:
Gemstones of the World, Schumann, W., Sterling Publishing Co., 2013.
Gemstones Symbols of Beauty & Power, Gubelin, E., Erni, F., Geoscience Press, Inc. 1999.
Gemological Institute of America (GIA) resources. International Gem Society (IGS) resources. Photos courtesy of the Smithsonian Geogallery.

Sakina Bharani is a gemologist and Museum docent.
 She has presented several lectures on gemstones.
Her lecture on Phenomenal Gems will be held on February 24, at 2 p.m. at the Museum. After the lecture join a guided tour of the Smithsonian Gems exhibit. Reservations are recommended.