Opalescence is the milky, pearly, sometimes bluish turbidity seen in common opal and in certain other materials whose internal structure scatters light at sub-wavelength scales. The phenomenon arises from Rayleigh or Tyndall scattering by submicroscopic silica spheres, voids, or other heterogeneities, and produces a diffuse white or bluish sheen that is distinct from the spectral play of colour in precious opal. The two phenomena are sometimes confused in casual usage, and their separation matters for both gemmological description and for the correct identification of materials. Opalescence is also observed in moonstone (where it is one component of the broader optical character of the variety), in some glass, and in colloidal solutions used in laboratory demonstrations of light scattering.

The physics

Rayleigh scattering, named after Lord Rayleigh's nineteenth-century work on the colour of the sky, describes the scattering of light by particles much smaller than the wavelength of visible light. The intensity of scattering scales with the inverse fourth power of wavelength, meaning that shorter wavelengths (blue end of the spectrum) scatter much more strongly than longer wavelengths (red end). This is why the cloudless sky appears blue: the atmospheric molecules scatter blue light more efficiently than red, and the scattered blue light reaches the observer from all directions while the red light continues largely undeflected.

Tyndall scattering, named after John Tyndall's contemporaneous work, describes a similar phenomenon for slightly larger particles approaching the wavelength of visible light, where the relationship is more complex but the broad pattern of preferential blue scattering still holds. In opal, the scattering centres are submicroscopic silica spheres or voids in the silica matrix; their size and distribution determine which wavelengths scatter most strongly and how diffusely.

The visible result in opalescent material is a soft, diffuse, often slightly bluish sheen that appears to come from within the body of the stone. The sheen is not the spectral, directional play of colour produced by diffraction in precious opal — opalescence does not show the characteristic flashes of red, orange, and green — but it is the optical character that gives common opal its distinctive appearance and the name from which the phenomenon takes its label.

Distinction from play of colour

The most important distinction in gemmological description is between opalescence — diffuse, scattering-based sheen — and play of colour — directional, diffraction-based spectral colour. Precious opal shows play of colour because its silica spheres are arranged in regular three-dimensional arrays at scales matching visible-light wavelengths, producing organised diffraction. Common opal shows opalescence because its silica is either disordered or sized incorrectly to produce diffraction, but is heterogeneous enough at submicroscopic scales to scatter light effectively.

The terms are sometimes used loosely in retail and in older literature, with 'opalescence' applied to anything from common opal sheen to fine play of colour to moonstone adularescence. Strict gemmological usage distinguishes the three: opalescence (Rayleigh scattering in common opal), play of colour (diffraction in precious opal), adularescence (the floating blue sheen of moonstone, caused by lamellar interference). See the separate entries on play of colour and adularescence for the related phenomena.

Where opalescence occurs

Common opal is the principal source of textbook opalescence. Common-opal varieties from Mexico, Peru, Brazil, Australia, and the United States all show varying degrees of milky to bluish sheen depending on the size and distribution of their internal scattering centres. Some Mexican and Peruvian common opal — pink opal, blue opal, dendritic opal — show pronounced opalescence and are valued for their colour and sheen even without play of colour.

Moonstone — the lamellar feldspar variety covered under the separate adularescence entry — shows a related but technically different phenomenon. Some glass, particularly historical milk glass and some art-glass formulations, shows opalescence by similar Rayleigh-scattering mechanisms when colloidal particles are dispersed in the glass matrix. Tyndall demonstrations with colloidal solutions in chemistry teaching are exact laboratory analogues of the same scattering process.

Documentation

The phenomenon is documented in standard gemmological references including the GIA gemstone curriculum, the Hurlbut and Klein Manual of Mineralogy, and the Gems & Gemology literature. Cornelius Hurlbut and Cornelius Klein's textbook treatment provides the standard physical-mineralogy reference, while GIA's curriculum addresses the gemmological description and the distinction from related phenomena. Lord Rayleigh's original papers from the 1870s remain the foundational physics reference, with Tyndall's contemporaneous work providing the complementary treatment for larger scattering particles.

In the trade

For working jewellers and gemmologists, the distinction between opalescence and play of colour matters for accurate description of material at sale, for laboratory reporting, and for managing customer expectations. A common opal cabochon described as opalescent should not produce customer disappointment when it does not show the spectral flashes of fine precious opal; the description sets the visual expectation correctly. Refractive index, specific gravity, and visual character together identify common-opalescent material reliably and distinguish it from precious opal, moonstone, and other related materials.

Care for opalescent common opal follows the standard opal protocol: avoid thermal shock, ultrasonic and steam cleaning, prolonged dehydration, and impact. The material is typically cabochon-cut and bezel-set rather than faceted and prong-set. See also opal, play of colour, and adularescence for related entries.

Further reading

• GIA — opal reference
• GIA Gems & Gemology — opal optical phenomena
• International Gem Society — opal and optical phenomena