Types of Iridescent Gemstones & Minerals

Iridescence is a optical phenomenon that causes a mineral to display a play of colors. It is caused by interference of light waves reflected from thin layers within the mineral. 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.

Iridescence can be seen in a variety of minerals, including labradorite, opal, moonstone, and aventurine quartz. It is also seen in some insects and birds, as well as in certain man-made materials, such as soap bubbles and CDs.

Types of Iridescent Gemstones & Minerals
Types of Iridescent Gemstones & Minerals

Types of Iridescence Gemstones & Minerals

There are several types of iridescence that have their own particular causes:
  • Labradorescence
  • Adularescence (or Schiller)
  • Aventurescence
  • Opalescence
Labradorescence Types of Iridescent Gemstones & Minerals
Polished labradorite

Labradorescence

Labradorescence (labradorite iridescence) is the effect seen in labradorite (a Feldspar) and Spectrolite (a Labradorite found in Finland). It is caused by the presence of thin layers of different minerals within the labradorite, which diffract light in different ways. The colors that are seen depend on the angle at which the light hits the stone and the thickness of the layers. Labradorescence can produce a variety of colors, including blue, green, yellow, orange, and red. Many Labradorites are carved to exploit this unique type of sheen.

iridescent moonstone - Blue sheen moonstone.
Blue sheen moonstone (Adularescence).
Photo: Crystalarium

Adularescence

This type of sheen is exhibited in Moonstone (another Feldspar), It is caused by the reflection of light between thin layers of different minerals within the stone. The layers are so thin that they are invisible to the naked eye, but they are thick enough to diffract light and produce the adularescence effect. 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. Aventurescence is most commonly seen in aventurine quartz, but it can also occur in other gemstones, such as sunstone, feldspar, and glass. The color of the aventurescence depends on the type and color of the inclusions. For example, aventurine quartz typically has a green or blue aventurescence, while sunstone has a red or orange aventurescence.

Opalescence play of color in Opal
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 ┬Ám (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.

An ammolite gemstone is a fossil ammonite.
Ammolite is a rare iridescent gemstone formed from the fossilized shell of Ammonite.
  Photo: Lou (@themineralcollective)

See also: 
How Do Asterism Minerals Form?
Why Fluorite Comes in Different Colors? With Examples
How Do Opalised Fossils Form?
What Is Cat's Eye Effect Observed in Some Minerals?
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