Gem of the Month

Each month this section will feature either a topic of interest to gem lovers or one special gemstone with background on the material and its value.

Go to: Homepage -- what's new in faceted gems -- what's new in designer cabochons and gem carvings -- gem of the month -- gem of the month archive -- birthstone of the month -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- about the ACS cutters -- settings for these gems --faceting information -- purchase UltraTec equipment

-- Mail to bsmigel@cox.net

{Search our Catalog}

January, 2003 {Revised, October, 2004}

Iridescent Gems

Due to their internal structure, a number of gem varieties show a surface or interior display of colors which are not part of the gems themselves, but rather created by the behavior of the light that enters them. This optical phenomenon, called iridescence, is familiar from everyday situations like the colored layer that oil or gasoline make on a puddle of water or the rainbow effect we might see on the surface of a soap bubble. Both diffraction and interference play a part in the effects we see.

In the case of color play in opals for example, the gem's ultrastructure based on uniformly packed spheres of cristobalite silica acts like a diffraction grating which breaks light into the various wavelengths of color. As they reflect, the now, slightly out of phase waves combine and subtract, by interference, and we see blocks of spectral colors. The size of the internal spheres determines the color: smaller spheres - we see blue, larger ones - we see red. Beyond a certain size, as in common opal, light doesn't have to bend to travel through the openings, so no color play is seen.

[Common opal: no color play / Precious opal: with color play]

In most of the other iridescent gems an ulrastructure of thin layers acts to create the color-making diffraction and interferences. One of the most highly valued gems of all time, pearl, gets its surface iridescence or "orient" from a subtle interference created as light travels through the thin layers of nacre. Mother of pearl and gem shell material, like abalone, have a more intense color display.

[Freshwater Pearl]

Two forms of feldspar are noted for their iridescent optical phenomena, moonstone and Labradorite. The adularescence or "shiller" of moonstone results from the internal layering of two interlaced feldspar varieties. The thickness of the layers controls color, or, in fact, whether we will see the phenomenon at all. When these layers are very thin the very desirable blue iridescence of gem blue moonstone results. In Labradorite and spectrolite a similar effect, which is confined to a single direction, is created from repeated internal crystal twinning.

[Moonstone / Labradorite]

Some of the most beautiful less common gems, owe their striking colors to iridescence as well. Ammolite, a fossilized shell, which shows iridescent colors over a brown or dark grey shell matrix is rapidly gaining popularity as is fire agate, a form of iridescent botryoidal chalcedony.

[Ammolites: blue and red]

[Fire Agate]

VALUE CONSIDERATIONS

All else held constant, the stronger the phenomenon of iridescence the more valuable the gem. In opals, larger patches of color, greater saturation of color and more individual colors are more desirable than tiny points or single colors. In pearls, a thicker nacre coating creates a more visible and even display of orient which increases their value. In moonstone, blue is the most valuable color, with the multicolors of fine rainbow moonstone close behind. Ammolites increase in value with the amount of red and especially blue and violet, in their displays, as green and gold are more common and the situation is very similar with fire agates.

The degree to which the phenomenon covers the entire surface is a strong value factor, with "dead" spots detracting substantially from value. Another factor is the degree of directionality of the phenomenon. Most Labradorites and some fire agates and ammolites, for example have single plane or a limited few angles at which strong colors show and at other angles this effect fades. As with any gem, body color, clarity, size and beauty of the fashioning are additional factors which influence price.

Gemological Properties:

Varies with species

Stones Currently Available:

{Search our Catalog} using "iridescent" or "iridescence" or by the particular species name of interest

-- Mail to bsmigel@cox.net