Opalescence Explained

Opalescence or play of color is the optical phenomenon displayed by the mineraloid gemstone opal,^[1] a hydrated silicon dioxide.^[2]

Definition

Each of the three notable types of opalprecious, common, and fire^[3] display different optical effects; therefore, the intended meaning varies depending on context.

• 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.^[4]
• 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.^[5] This milky sheen displayed by opal is a form of adularescence.
• 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 (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 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.^[6]

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, 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.

See also

• Aventurescence
• Labradorescence
• Moonstone (gemstone)

Notes and References

1. 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
2. Web site: Opal Gemstone Information . 2018 . Gemstone Select . 2019-01-08.
3. Douma, M., curator. 2008. Opal. In Cause of Color. Retrieved January 8, 2019, from https://webexhibits.org/causesofcolor/15F.html
4. 2019-01-08.
5. Web site: Opal Description . 2019 . . 2019-01-08.
6. 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