Types of Iridescent Gemstones & Minerals

Iridescence is the play of color, or a series of colors, produced by interference or diffraction (or both), either when light is reflected from thin films (inclusions), twinning planes or from the unique structure of precious opal.

There are several types of iridescence that have their own particular causes:

Labradorescence
Adularescence (or Schiller)
Aventurescence
Opalescence

Polished labradorite

Labradorescence

Labradorescence is the effect seen in Labradorite (a Feldspar) and Spectrolite (a Labradorite found in Finland). It is caused by interference on the boundaries of lamellar twin planes, which are usual in Feldspars. Many Labradorites are carved to exploit this unique type of sheen.

Unpolished moonstone. Photo: Crystalarium

Adularescence

This type of sheen is exhibited in Moonstone (another Feldspar), caused by reflection on Moonstone's lamellar twinning planes. The effect is a blue color floating just below the surface of the stone. Adularescence is also known as Schiller.

Rainbow pyrite Types of Iridescent Gemstones & Minerals

Rainbow pyrite, a variety of mineral pyrite. The iridescence is caused by tarnishing by oxidation and molybdenum traces in the mineral. Photo: Ryan lay

Aventurescence

Aventurescence is named after Aventurine Feldspar, which is also known as Sunstone. This type of iridescence is due to the play of color caused by reflection on tiny, thin inclusions of goethite and hematite (or both). This gives the stone a golden or reddish-brown color and specular reflections.

Stunning Australian opal! Photo: Able Ground

Opalescence

The causes of play of color in Opal were long uncertain until the invention of the electron microscope. This enabled scientists to see the unique structure of Opal at high magnification, and to discover that Opal is made up of small spheres of silica.

In Opal, both interference and diffraction play a role in the play of color. Interference occurs when part of the light gets reflected from the surface of a silica sphere while another part gets refracted inside the sphere, being reflected again. Diffraction in Opal is the result of light hitting a gap between the spheres and then being split up into its spectral components.

In precious Opal, larger silica spheres of about 350 µm (micro meters) in diameter give off red flashes with changes in viewing angles. The smaller spheres result in green, blue or purple flashes which cannot increase in wavelength to give a red flash. Therefore, the sizes of the 'gaps', or 'voids', determine which color is seen.