![Opal-53714](https://upload.wikimedia.org/wikipedia/commons/f/fa/Opal-53714.jpg)
Opalescence refers to the optical phenomena displayed by the
mineraloid gemstone opal
Opal is a hydrated amorphous form of silica (SiO2·''n''H2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline form ...
[opalescent. 2019. In Noah Webster's 1828 American Dictionary of the English Language. Retrieved January 7, 2019, from https://1828.mshaffer.com/d/word/opalescent] (hydrated silicon dioxide).
[Opal Gemstone Information. 2018. In Gemstone Select. Retrieved January 8, 2019, from https://www.gemselect.com/gem-info/opal/opal-info.php] However, there are three notable types of
opal
Opal is a hydrated amorphous form of silica (SiO2·''n''H2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline form ...
(precious, common, and fire),
[Douma, M., curator. 2008. Opal. In Cause of Color. Retrieved January 8, 2019, from http://www.webexhibits.org/causesofcolor/15F.html] each with different optical effects, so the intended meaning varies depending on context:
* Precious opal. The general definition of opalescence is a milky
iridescence displayed by an opal which describes the visual effect of precious opal very well, and opalescence is commonly used in lay terms as a synonym for
iridescence.
* Common opal. In contrast, common opal does not display an
iridescence but often exhibits a hazy sheen of light from within the stone—the phenomenon that gemologists define strictly as opalescence.
["Opal Description". 2019. Gemological Institute of America. Retrieved January 8, 2019, from https://www.gia.edu/opal-description.] This milky sheen displayed by opal is a form of
adularescence
Adularescence ( ) is an optical phenomenon that is produced in gemstones like moonstone. The optical effect is similar to labradorescence and aventurescence.
Description
The effect of adularescence, also commonly referred to as '' schiller'' ...
.
* Fire opal is a relatively transparent gemstone with a vivid yellow-orange-red color and rarely displays iridescence.
The optical effects seen in various types of opal are a result of
refraction
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenome ...
(precious and fire) or reflection (common) due to the layering, spacing, and size of the myriad microscopic silicon dioxide spheres and included water (or air) in its physical structure.
When the size and spacing of the
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
spheres are relatively small, refracted blue-green colors are prevalent; when relatively larger, refracted yellow-orange-red colors are seen; and when larger yet, reflection yields a milky-hazy sheen.
[Smigel, Barbara W. 2012. Optical Phenomena in Gemstones. In Introduction to Gemology. Retrieved January 8, 2019, from http://www.bwsmigel.info/Lesson6/DE.Optical.Phenomena.html]
![Why is the sky blue](https://upload.wikimedia.org/wikipedia/commons/0/0b/Why_is_the_sky_blue.jpg)
In a physical sense, some cases of opalescence could be related to a type of
dichroism seen in highly
dispersed systems with little
opacity. Due to
Rayleigh scattering
Rayleigh scattering ( ), named after the 19th-century British physicist Lord Rayleigh (John William Strutt), is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of th ...
, a transparent material appears yellowish-red in transmitted white light and blue in the scattered light perpendicular to the transmitted light.
The phenomenon illustrated in the bottom photo is an example of the
Tyndall effect
The Tyndall effect is light scattering by particles in a colloid or in a very fine suspension. Also known as Tyndall scattering, it is similar to Rayleigh scattering, in that the intensity of the scattered light is inversely proportional to ...
.
See also
*
Aventurescence
In gemology, aventurescence (sometimes called aventurization) is an optical reflectance effect seen in certain gems. The effect amounts to a metallic glitter, arising from minute, preferentially oriented mineral platelets within the material. Thes ...
*
Labradorescence
Labradorite (( Ca, Na)( Al, Si)4 O8) is a calcium-enriched feldspar mineral first identified in Labrador, Canada, which can display an iridescent effect (schiller).
Labradorite is an intermediate to calcic member of the plagioclase series. It ...
*
Moonstone (gemstone)
Moonstone is a sodium potassium aluminium silicate ((Na,K)AlSi3O8) of the feldspar group that displays a pearly and opalescent schiller. An alternative name is hecatolite.
Etymology
File:Pierrelune.jpg, Moonstone polished ''en cabochon''
T ...
References
{{Reflist
Optical phenomena