Gemstone Phenomena

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Cat’s Eye Chatoyancy

This optical effect is used to describe gemstones that reflect light along a narrow band across the center of the gemstone. The term comes from the French for ‘Eye of Cat’, which is apt as the beam of reflected light across the surface of the gem can resemble its namesake. The effect can be caused either by the fibrous nature of the material’s structure, or by fibrous inclusions within the gemstone. If these fibers are parallel to one another, then the gemstone will display Cat’s Eye Chatoyancy (and if the fibers a disorganized, the gemstone will display Simple Chatoyancy as described below). In order to maximize the effect, a gemstone generally needs to be cut en cabochon.

When a variety of gemstone exhibits this quality, Cat’s Eye is often used as a prefix or suffix – Cat’s Eye Quartz, for example. Sometimes, a gem itself is referred to simply as ‘Cat’s Eye’ – in this case, the term describes Chrysoberyl Cat’s Eye specifically.

Gemstones known to display Cat’s Eye Chatoyancy include:

  • Chrysoberyl Cat’s Eye (Probably the variety of gemstone most associated with this effect)
  • Quartz Cat’s Eye
  • Cat’s Eye Aquamarine
  • Cat’s Eye Tourmaline
  • Cat’s Eye Apatite
  • Cat’s Eye Moonstone
  • Cat’s Eye Scapolite
  • Actinolite Cat’s Eye
  • Cat’s Eye Diaspore

Simple Chatoyancy

Simple Chatoyancy is similar to Cat’s Eye Chatoyancy, but occurs in gems when the fibrous material inside the stone is disorganized rather than parallel. The resulting effect is that a gemstone will show similar sheen, but the reflected light won’t be focused in a line through the center of the gemstone. In this case, the whole surface of the gemstone normally exudes a glossy, silk-like quality. Simple Chatoyancy is seen in less gem varieties than Cat’s Eye, and the effect is less pronounced, so it is less well known and is sometimes lumped together with the Cat’s Eye variant.

Gemstones with Simple Chatoyancy include:

  • Tiger’s Eye (A form of Quartz)
  • Pietersite
  • Seraphinite
  • Charoite
  • Sapphire (rare)
  • Ruby (rare)


Asterism, though technically just another form of Cat’s Eye Chatoyancy, is often given its own name due to the remarkable effect it creates in gemstones. Gemstones with Asterism have a stunning star like effect across their surface. Asterism occurs when, rather than having aligned fibers in just one direction, a gemstone has fibers aligned on two or more axes. The reflected light effect then runs along these same axes. Like Cat’s Eye Chatoyancy, for gemstones to fully display their Asterism, gemstones must be cut en cabochon. Gemstones that display Asterism often carry the prefix or suffix Star – for example, Star Sapphire would be Sapphire that displays Asterism.

Gemstones with Asterism include:

  • Star Garnet
  • Star Rose Quartz
  • Star Sapphire
  • Star Moonstone
  • Star Ruby
  • Star Diopside
  • Star Chrysoberyl


Adularescence is the effect where a gemstone appears to have a light coming from the inside, causing the gemstone to seem to glow. Of course, we know that this isn’t actually the case (try turning off the lights with an Adularescent gem in the room. It will remain dark). Rather, this effect is caused by the micro structure of a gemstone. Because certain gemstones have layers of different materials interlaced through their bodies, light can travel at different speeds through the layers, causing the light to scatter and producing the Adularescent effect. Moonstone is the gemstone most associated with this effect, and the term ‘Adularescent’ comes from the word ‘Adularia’, which is a mineral group that Moonstone falls into. While white Adularescence is most common, in moonstones it is possible for the ‘light’ within the gemstone to exhibit a blueish or orangeish tint, creating an incredibly attractive effect. However, these occurrences can be rare and gemstones that show this colored Adularescence are often quite valuable.

Adularescence can have other names – it is sometimes also known as Schiller. In Opals, this effect is often called opalescence, and is normally more subdued than in other gemstone varieties and produces a milky, hazy effect.

Gemstones known to display Adularescence

  • Moonstone
  • Opal
  • Quartz
  • Agate


Iridescence is the rainbow effect that is seen is some gemstones – the shimmering of various colors across the surface of a stone. It is caused by diffraction of light within a gemstone – different layers of material can create a prism-like effect, causing the light to split into its component colors. The most famous example of iridescence is probably precious opal (although often this effect in opals is wrongly credited as opalescence), which can display a dazzling array of colors. Iridescence is actually not all that uncommon in gemstones. In fact, we even occasionally see it in everyday life – the back of a CD or the surface of a bubble can also exhibit this effect. Sometimes this effect is also known as play of color, particularly with regards to Opals. Labradorescence is also a form of Iridescence, but is used exclusively to describe the effect in the Labradorite mineral. Specifically, Labradorescence describes cases where the effect is extremely directional, causing crystals to look dull from some angles, but reveal a dazzling array of colors as it is viewed from other angles.

Some gemstones that display Iridescence include:

  • Precious Opal
  • Fire Agate
  • Ammolite
  • Rainbow Pyrite
  • Pearl
  • Obsidian
  • Labradorite


Pleochroism in Tourmaline Crystal

Pleochroism describes when a gem appears to be different colors when viewed at different angles. The effect is often classified as weak, distinct, or strong, and in the strong form the effect can be quite astounding – almost like the gem is genuinely changing color. The science behind it is a tad complex – in essence, the effect is caused when a gemstone has a structure where light entering the gem from certain angles is absorbed at different wavelengths. There are cases of pleochroism where a gem can display three different hues – in this case, it is also known as trichroism. In addition, two-color pleochroism can be called dichroism. The term derives from the Greek words for ‘More Color’. Pleochroism is sometimes mistakenly referred to as color change, but color change is in fact a separate optical effect.

Since there are many varieties of gemstones that display weak or unremarkable pleochroism, we have only listed only some of the more notable cases.

  • Andalusite
  • Kunzite
  • Iolite
  • Alexandrite
  • Sphene
  • Tanzanite (in its natural form, it loses the effect after heat treatment)
  • Hiddenite
  • Corundum
  • Zoisite
  • Diaspore
  • Elbanite

To see the remarkable effect in action, you can check out the video below:

Color Change

Color Change in Alexandrite

Color Change is the term used to describe gems that display different colors depending on whether they are under sunlight (or equivalent) or incandescent light. This remarkable effect occurs because daylight contains more electromagnetic waves that are closer to the blue end of the spectrum, whereas incandescent or lights contain more red. If a gemstone absorbs all but two wavelengths of light – lets say red and blue, for example – Then under daylight, much more blue will be reflected, and under incandescent light, much more red will be reflected, causing the gemstone to display color change. This optical effect is also known to some as the Alexandrite effect, and Alexandrite is famous for its color changing properties. Individual gemstones that show this effect will have the prefix color change – e.g Color Change Garnet – and only specific specimens of each type of gemstone listed below will display the effect.

  • Alexandrite (famous for this effect)
  • Color Change Garnet
  • Color Change Sapphire
  • Color Change Diaspore
  • Color Change Spinel
  • Color Change Tourmaline

To see this effect in action, you can take a look at the video below:


Aventurescence describes the effect where a gem can seem to sparkle, almost like the effect that glitter has. This is caused by miniscule inclusions (e.g copper) in a gemstone, distributed randomly, all reflecting light at once. The effect is named after Aventurine Quartz, which is known for displaying this effect, but can also occur in other varieties of gemstone. The term shiller, which can refer to Adularescence, can also refer to Aventurescence (more or less the term shiller can just be used for anything shiny).Please note, the word ‘Aventurescence’ has no D within it – and the term is not ‘Adventurescence’, as, much to our chagrin, we find gemstones rarely take part in adventures.

There are only a few gemstone varieties that are known to exhibit Aventurescence, they’re listed below:

  • Aventurine Quartz
  • Sunstone
  • Oligoclase
  • Andesine-Labradorite
  • Gold-sheen Obsidian
  • Bloodshot Iolite


Tenebrescence in Hackmanite

Tenebrescence describes the phenomenon where a gemstone can change color when exposed to sunlight and revert back to its original color. The effect is also known as photochroism. this effect differs from Color Change because a Tenebrescent gemstone exposed to sunlight changes color and then remains this color for a period of time. The effect can be likened to the special glasses that become shaded sunglasses when worn in daylight. This effect is relatively rare and only occurs in a handful of gemstones. There are some minerals and gemstones that change color permanently (irreversibly) when exposed to sunlight – although some may refer to this as photochroism, we believe this to be an incorrect usage of the term.

Gemstones that exhibit Tenebrescence include:

  • Hackmanite Sodalite
  • Tugtupite
  • Scapolite

To see Tenebrescence in action, you can watch this video:

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