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OPTICAL PHENOMENA (PHENOMENONS) IN CRYSTALS…

Crystal Optical Phenomena is an effect produced when a light source interacts with the structural features of or the inclusions within the crystal.  It involves the transmitting or reflecting of light that it is subjected to. Generally, these phenomenons ARE NOT a result of the crystal’s chemical make-up.

Optical effects are sometimes named after a crystal or its mineral content. ie: opalescence (opals), aventurescence (aventurine), labradorescence (labradorite) and adularescence (adularia, a type of moonstone).

One term used to describe an optical phenomena is “The Girasol Effect.” The girasol effect refers to a glowing milky sheen that appears like it is moving when the stone is turned in the presence of any light source.

“Girasol” in Spanish means “sunflower.” Girasols can refer to hyacinth opals, a yellow or orange type of precious opal OR semi-transparent opals which have a bulgy bluish sheen similar to moonstones.

With hyacinth opals, a light appears to float within the crystal.  This floating light follows the light source it is exposed to.  This is different from opalescence in common opals which displays a subtle haziness with the scattering of light.

Here are some common crystal optical phenomena….

ADULARESCENCE comes from the word “adularia,” a translucent or transparent orthoclase mineral.  Moonstones are a member of the orthoclase family and adularescense applies primarily to moonstones.  Besides moonstones, adularescence is found in agates, milky white opals and rose quartz.

Adularescence is caused when light interacts and scatters the tiny inclusions within the crystal.  In the case of moonstone, the light source affects the alternating layers of orthoclase and albite.

This phenomenon appears like moon light floating on water, hence the name “moonstone.” The generally milky white / bluish glowing sheen (usually non hazy) originates from below the surface of the gemstone. Turning the stone or removing it from a light source makes the “effect” appear like it is moving.  White adularescence is more common, however orange or blue lusters can appear in the more rare stones.

The term “schiller or “shiller” (German for twinkle) is often used interchangeably for adularescence or aventurescence.  It refers to the metallic iridescence that originates below the surface of the crystal when light is reflected between the layers of minerals.

ASTERISM is a popular phenomena. The word “asterism” comes from the Greek word “aster” which means “star” and the effect created is a luminous wavy star shape across the surface of the stone. This often billowy and shiny looking reflection is caused when light hits and scatters the tiny parallel shaped fibers inside the crystal. As the stone is turned toward or away from a light source, the reflection appears as if it is floating or moving. The inclusions are often rutile impurities which can be in the shape of needles, tubes or platelets.

Asterism is best seen when a stone has been cut into a cabochon and is exposed to direct sun light.  Stones that may display asterism are star rubies, star sapphires, garnet, diopside, rose quartz and spinel.

There are two types of asterism…

Epiasterism refers to light being reflected due to the parallel shaped inclusions inside the stone. Stones in this category are rubies, sapphires and most other gems.

Diasterism refers to light that is transmitted through the stone. This occurs when the light is shone through and illuminated from the bottom of the stone.  Rose Quartz is in this category.

AVENTURESCENCE (AKA  aventurization) is a reflective effect that looks like glitter.  It comes from many tiny MINERALS WITHIN the stone. In the case of green aventurine, it is the mica particles of fuschite that turn it green.  Iron oxides make aventurine red.  Sunstone is another stone that exhibits aventurescence.

CHATOYANCE or CHATOYANCY (“oeil de chat” is French for cat’s eye)

These are clearly defined silky looking luminous bands of light that run length wise from one end of the stone to the other. They appear to “float” or turn when the crystal is rotated. A cat’s eye effect is created when the parallel fibrous INCLUSIONS or CAVITIES within the stone reflect light into a direction perpendicular to their direction.

Chatoyance is found in chrysoberyl cat’s eye, aka cymophane, Greek for “wavelike form.”

Chatoyance is also displayed in tiger eye.  In tiger eye, the bands are caused by the fibrous STRUCTURE of the material.  Other stones which can display a similar chatoyant effect to chrysoberyl are tourmaline, aquamarine, scapolite, spinel, quartz and ruby.

FLUORESCENCE is caused when invisible short wavelengths are absorbed within the crystal and are then converted and sent back out as longer wavelengths. When these fluorescent minerals are exposed to ultraviolet radiation light, they give off an invisible light.  Usually, an impurity within the crystal is responsible for its fluorescence and the most common impurity in fluorescent minerals is manganese.

Many crystals have fluorescence.  Some of the more common ones are apatite, aragonite, beryl, calcite, celestite, danburite, dumortierite, fluorite, hiddenite, howlite, kunzite, prehnite, rhodocrosite, rhodonite, selenite, sodalite, sphalerite, spinel, sulphur and willemite.

IRIDESCENCE in crystals is when the surface appears to change color as the angle of the view or illumination from the light changes. The word iridescence comes from “iris” which in Greek means “rainbows.”

3 Types of Optical Iridescence (Orient and Labradorescence and Play of Color)

ORIENT IRIDESCENCE  The word “orient” refers to the deep and warm lustrous glow that are found in pearls.  In addition to pearls, fire agate, rainbow calcite, some obsidians and iris agate belong to this classification.

LABRADORESCENCE: is an optical phenomenon found in labradorite.  Labradorite is a feldspar composed of a complex range of minerals and very fine grained layers of sedimentary rock.  Labradorite has thin fractures within the stone which appear on the surface.  When light hits the parallel layers and fractures within the stone, it can create iridescence flashes of blues, greens, oranges, yellows and reds.  This is particularly true with grayish to black varieties of labradorite.

PLAY OF COLOR (aka Color Flashes or Pseudochromatic Coloration) can be caused a couple of ways.

Flashes can be caused when rays of light are scattered and broken down by different light wavelengths.  When the angle of the light source changes, so do the colors that are reflected from the crystal’s surface.

These color flashes happen when some of the light is reflected from the surface of the stone while some of the light is reflected off the backside of the stone as a result of the light travelling through the crystal.

Stones that have a play of color include ammonite, cat’s eye, feldspar, labradorite and opal.

OPALESCENCE (aka Defraction) appears as a milky iridescence or brightness from the surface of the crystal. This subtle hazy scattering of light is most commonly found in common opals and in some agates.

PEARLESCENCE: (aka Luster) is another form of irisdescence.  Pearlescence displays lustrous rainbows.  This pearly appearance can be found primarily in pearls, talc and gypsum.

PLEOCHROISM (aka COLOR CHANGE EFFECT and TENEBRESENCE) This optical phenomenon causes a crystal to change colors when it is viewed at different angles with various light waves.

Some crystals change colors depending on the light source they are subjected to: sun light, fluorescent light or incandescent light.

Alexandrite is known for its strong pleochroism properties..  Alexandrite, a chrysoberyl, is considered the best example of a stone that changes colors.  One reason for its green to red color change is the type of light it is exposed to.  Alexandrite that comes from the Ural Mountains in Russia is green by natural sun light and red by incandescent light (the light given off by our standard light bulbs).  Other varieties of alexandrite can be yellowish or pink in daylight and grayish tan to raspberry red by the common incandescent light bulb.

Another reason for its color change could be a very slight change in its mineral content. Alexandrite contains aluminum, iron, titanium and chromium. Minute amounts of its aluminum are sometimes replaced by chromium which can explain another reason for its color change. This is known as the “alexandrite effect.”

Other gems that can change color are: blue garnets, sapphires, spinel and rubellite (pink tourmaline).