Opalescence or play of color is an

optical phenomenon Optical phenomena are any observable events that result from the interaction of light and matter. All optical phenomena coincide with quantum phenomena. Common optical phenomena are often due to the interaction of light from the Sun or Moon with ...

associated with the

mineraloid A mineraloid is a naturally occurring substance that resembles a mineral, but does not demonstrate the crystallinity of a mineral. Mineraloid substances possess chemical compositions that vary beyond the generally accepted ranges for specific mi ...

gemstone

opal Opal is a hydrated amorphous form of silicon dioxide, silica (SiO2·''n''H2O); its water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to the amorphous (chemical) physical structure, it is classified as a ...

,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 a

hydrate In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...

silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundan ...

. This effect appears as a milky, translucent glow that changes with the angle of light, often creating a soft, pearly sheen that can display various colors or hues. Opalescence can be seen in materials like certain minerals, glass, and even fluids.

Definition

Each of the three notable types of opalprecious, common, and fireDouma, M., curator. 2008. Opal. In Cause of Color. Retrieved January 8, 2019, from https://webexhibits.org/causesofcolor/15F.htmldisplay different optical effects; therefore, the intended meaning varies depending on context. * The general definition of ''opalescence'' is a milky

iridescence Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear gradually to change colour as the angle of view or the angle of illumination changes. Iridescence is caused by wave interference of light in microstru ...

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''. * In contrast, common opal does not display an iridescence, but often exhibits a hazy sheen of light from within the stonethe phenomenon that gemologists strictly term as ''opalescence''. 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'' o ...

. * Fire opal is a relatively transparent gemstone with a vivid yellow-orange-red color and rarely displays iridescence.

Mechanism

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 transmission medium, 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 commo ...

(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 , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

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

In a physical sense, some cases of opalescence could be related to a type of

dichroism In optics, a dichroic material is either one which causes visible light to be split up into distinct beams of different wavelengths (colours) (not to be confused with Dispersion (optics), dispersion), or one in which light rays having different P ...

seen in highly dispersed systems with little

opacity Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shie ...

. Due to

Rayleigh scattering Rayleigh scattering ( ) is the scattering or deflection of light, or other electromagnetic radiation, by particles with a size much smaller than the wavelength of the radiation. For light frequencies well below the resonance frequency of the scat ...

, 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 such as a very fine suspension (a sol). Also known as Tyndall scattering, it is similar to Rayleigh scattering, in that the intensity of the scattered light is inversely propor ...

Fluids

In fluids, the near the gas-liquid transition, the substance can become cloudy. This phenomenon is called

critical opalescence In physics, critical opalescence refers to the dramatic increase in scattering of light in the region of a continuous, or second-order, phase transition. Near the critical point, the properties of the liquid and gas phases become indistinguishabl ...

References

{{Reflist Optical phenomena

Definition

Mechanism

Fluids

See also

References