How does ‘Phenomena Effects’ like Adularescence, Asterism, Chatoyancy, Iridescence, Play of Color, and Color Change affect the value of gemstones?

In our previous blog posts, we have discussed how to determine the value of colored gemstones.  In summary, colored gemstones value affected by six factors.

  • Color
  • Carat Weight
  • Clarity
  • Cut
  • Origin
  • Treatment

However, certain gemstones actually exhibit different phenomena effects like

  • Adularescence
  • Asterism
  • Chatoyancy
  • Iridescence
  • Play of Color
  • Color Change

So, “How do these gemstones get these phenomena and how does it affect the value?” This is what I will answer in today’s blog post.

What causes phenomena in gemstones?

Phenomena gems made either

A) by a gemstones ‘growth or material’ structure or

B) by having special inclusions.

Phenomena gems appear on all levels in the gemstone market – from high to low end. Like regular gemstones, a phenomena gemstones’ value is determined by rarity and desirability.

Example of most valuable phenomena gemstones are:
a) Top grade star corundum (rubies or blue sapphires)
b) opals
c) alexandrite
d) cat’s eye chrysoberyl

More abundant gemstones like Moonstones and labradorite are desirable but less valuable.

Others like aventurine quartz are plentiful and inexpensive.

A. PHENOMENA CAUSED BY STRUCTURE

Moonstone, labradorite, opal, some pearls get their phenomena color by their gem’s structure. These gems are made of small, minute, repeated layers.

When light rays enter these layers, they either ‘reflect or refract’ to varying degrees. The rays’ interaction with each other produces these unique optical effects. These interactions are called interference.

Interference definition is exactly what it sounds like. Different light waves interfering with each other.

If two light waves are parallel or synchronized. One light waves peak and troughs corresponding exactly to other’s peaks and troughs. They reinforce each other, which make brighter colors.

If the light waves are exactly opposite to each other. Each peak of one wave occurring in exactly the opposite trough of the other wave. They cancel each other, resulting in darkness.

PHENOMENA GEMSTONES – PLAY-OF-COLOR

Diffraction is a special type of interference phenomenon. Instead of bright or dark areas, this phenomenon produces pure colors like red, yellow, orange, green, blue, or violet. This happens because the stone has different layers of alternating materials. Which makes the light interact with different Refractive Indexes (RIs).

For opals, diffraction is the most important phenomenon. If you magnify opal’s structure with a powerful microscope, you would see millions of tiny spheres (called silica). This silica stacked in layers to give a regular three-dimensional arrangement. What holds this structure together is some different kind of silica and water. This small difference in RI between spheres causes the light to diffract which called play-of-color. Smaller spheres produce blue to green play-of-color and larger spheres produce red.

phenomena gemstones - side profile of rough opal crystal showing play-of-color
Multi-color rough crystal opal from Coober Pedy, South Australia, showing almost all color spectrum (blue, green, red, yellow) © Wiki Dpulitzer

Opal actually comes in many different quality grades. Thus has different ‘price levels’ in the gemstone market. Opal is one of the most popular phenomenon gems.

You can read more about opals here:
Natural Opal buying guide – How to check and understand difference between different opals?

PHENOMENA GEMSTONES – IRIDESCENCE

Iridescence is one of the most common interference phenomena. It happens when light passes through a thin transparent film that is different RI from its surrounding material. Examples of this effect is seen in soap bubbles or from thin layers of oil mixed with water.

The colors come from different reflection points.  Some from the top of the film while others that penetrate through the film reflect from the bottom.  Depending on RI and thickness of the film, light waves are either traveling together or out-of-phase. As learned earlier this causes the color to either be brighter or darker.

Most famous gemstones who have this effect are the Pearl Nacre also known as the mother of pearl. The gem made of two different materials: aragonite and conchiolin. In the trade, the pearl iridescence effect has a special term called ‘orient.’

Phenomena gemstones - Iridescence effect - Image of mother of pearls which display iridescence
Mother of Pearls lot, which my grandfather acquired in 1960s.  Though not 100% clear in this image these stones do display “iridescence” phenomena

PHENOMENA GEMSTONES – ADULARESCENCE

This phenomenon is something we see every day in the morning, the beautiful “blue sky.” The blue color made of air molecules scattering the sun’s light by deflecting it in many different directions.

This same effect happens in Moonstone. The most famous adularescence gemstone which has a similar blue sheen. Moonstone’s structure isn’t regular. It is actually made up of alternating layers of feldspars, which vary in thickness and regularity. These variations cause the incoming light waves to scatter and thus makes ‘blue sheen.’

B. PHENOMENA CAUSED BY INCLUSIONS

In general, inclusions usually take away from a gem’s beauty. The more inclusions there are, the lower is the value of a gemstone.

Yet, there are some inclusions of which increase the value of the gem. If it is ‘the right type’ or in ‘the right position’ it can cause spectacular optical effects.

These inclusions can produce by the reflection of the light rays, a ‘cat’s eye’, a ‘star’ or provide glittery sparkle.

PHENOMENA GEMSTONES – CHATOYANCY

The most famous ‘chatoyancy’ gemstone is the cat’s eye chrysoberyl. The gem’s striking “eye” is from parallel needle-like inclusions of fine hollow tubes. When the rough cut correctly, it reflects the light to create a silky sheen called chatoyancy.

Cutters produce the best cat’s eye gems by cutting the cabochon’s base parallel to gem’s inclusions. This cutting style concentrates the sheen across the dome of the cabochon.

In addition to chatoyancy, top-grade cat’s eye chrysoberyl shows ‘milk and honey’ look. When the light source is set at the right angle, this effect can be seen. Thus, the cat’s eye one side looks milky while the other retains the honey look.

Other gemstones like Tourmaline or quartz can also chatoyancy. In fact, I discovered that even tanzanite has it. If the right type of inclusions is in the stone, it can also display cat’s eye. This is exceptionally rare and I was quite shocked by this discovery. But, in the end, no gemstone can match the fine cat’s eye chrysoberyls appearance or value.

How to determine the value of cat’s-eye chrysoberyl?

Factors that affect the value of cat’s-eye chrysoberyl are:
a) Quality of its phenomenon
d) Body-color
c) Transparency
d) Quality of cutting.

Summary of how the value of the cat’s eye ranges from least expensive to most expensive are provided below.

Least expensive quality

  • Incorrectly oriented poorly focused eyes.
  • They also have surface imperfections and prominent inclusions that disrupt clarity.
  • Gems might also be almost opaque or so transparent that it’s hard to see any chatoyancy at all.
  • Body color might be an unattractive brown or gray and the quality of the cut might be poor.
  • Many stones are irregular, unsymmetrical cabochons with too much weight below the girdle.
Phenomena gemstones - Chatoyancy - Cat's eye 4.5 mm chrysoberyl lot
4.5 mm cat’s-eye chrysoberyl which I came across in JTC Bangkok. These are the least expensive quality as they have all the issues stated above.

Mid-priced quality

  • Usually display correctly oriented cat’s-eyes
  • Good body color with some evidence of milk and honey
  • Gems should have much better clarity and be free of surface imperfections.
  • Some cutting variations are possible, but most gems should have reasonable proportions overall.

Best (High priced) quality

  • Best cat’s-eyes must have sharp, well-focused, correctly oriented eyes
  • Fine body color with clearly evident milk and honey effect.
  • The most sought-after gems have a balance between the material’s transparency out in sharp relief against the stone’s body color.
  • The best cabochons are symmetrical, nicely proportioned and free of surface imperfections.

PHENOMENA GEMSTONES – ASTERISM

Asterism occurs in corundum crystal (both rubies and sapphires). The stones have three sets of rutile needles intersecting each other ‘three-dimensionally’ to form a six-rayed star.

This phenomenon formation is like chatoyancy. The difference is in the cat’s eye there is only one parallel needle inclusion. While in asterism there are three sets intersecting each other.

Several gem species like garnet, diopside and spinel show asterism.  But, the most valuable are star sapphires or rubies.  The best star corundum is semi-transparent which is very rare.

Most star rubies and sapphires are translucent or opaque.  Value determine by the color of the gemstone and by ‘how sharp and where the position of the star’ is in the gemstone.

To show ideal asterism, cutters try to place the reflective inclusions parallel to the base of the cabochon.  Ideally, the star should be sharp and distinct with straight, even rays intersecting at the top middle part of the stone.

How to Evaluate Star Corundum Value

Phenomena Gemstones - Asterism - image of six-rays 18.87 carats premium star blue sapphire image
18.87 carats blue sapphire we recently sold to a client. Notice the amazing translucent vivid blue color and “almost” distinct six-rays

When looking at star corundum, you first will have to examine the stone under the spotlight. Because diffused fluorescent lights won’t show the star.

Then while pointing the light source, rock the gem back and forth to see how the star moves across the gem’s surface. To determine if the gem is symmetrically cut, try spinning it on a flat surface. (Without dropping it of course!)

Judging the star

  • Is the star distinct, well centered and uniform?
  • Are any of the star’s rays incomplete?

Judging the cut

  • Are the gem’s proportions attractive and symmetrical?
  • Does the gem have excess weight below the girdle?
  • Do inclusions disrupt the gem’s polish or star?

Judging the color

  • Is the gem’s body color attractive?
  • Is the gem semi-transparent, translucent, or opaque?

*26.84 carat Star Blue Sapphire – notice that below girdle has excess unsymmetrical weight. Star not completely ‘distinct’ six rays and blue color shade is slightly lighter color. All these factors are taken into account when purchasing star sapphire (or corundum)

AVENTURESCENCE

Aventurescence occurs when gemstone sparkles with thousands of tiny reflections (simple glittery effect). This phenomenon is caused by light reflecting from many small, eye-visible, plate-like inclusions in the stone. When the stone rotated in the light these inclusions reflect the light back to the viewer.

An example of aventurescence gemstone is sunstone feldspar. Copper or hematite inclusions causes the aventurescence. This makes the gem have that amazing ‘golden color.’

In aventurine quartz, spangles of green mica produce the glittery effect and also give the gem its color.

PHENOMENA GEMSTONES – COLOR CHANGE

The most famous color change gemstone which buyers want is alexandrite chrysoberyl. Top grade ones are usually featured in exclusive auction catalogs. The gemstone got famous by romantic history of Russian czars prizing it. As well as its fascinating color change phenomenon.

In alexandrite, the crystal has chromium which features in both ruby and emeralds. That is why they can portray both red and green colors. What makes alexandrite unique is it allows equal amounts of red and green light to pass through it. And absorbs light in the middle of the visible spectrum.

Alexandrite’s color varies based on the type of lighting you view the gemstone under. When alexandrite is under lighting that is rich in red like incandescent or yellow light bulb the gem looks like a ruby. If it’s under lighting that’s rich in blue and green like a fluorescent white light the gem looks like an emerald. This phenomenon is called color change.

But, even the best alexandrite can’t match the finest ruby or emerald in color. It’s the gem’s striking red to green color change that draws people in wanting to buy these very unique stones.  Alexandrite is the most valued color-change gem.  Other color change gemstones are sapphire, spinel, and garnet.

How to determine value of alexandrite?

Phenomena Gemstones - Change of Color - image of 2.10 carat Alexandrite from Orissa, India
2.10 carat Alexandrite from Orissa, India. Stones from this mine usually don’t have strong color change.

Alexandrite hue range is summaraize below:
a) Strong bluish-green through yellowish green in daylight
b) Orangey red through purple-red in incandescent light

Commercial-quality alexandrite tone tends to be either too dark or light. It also has weak color saturation. For this reason, the color-change difference between the duller colors is less noticeable.

Finest-color alexandrite have dramatic color change. Green to bluish-green in daylight and red to purplish-red in incandescent light. Also, it has a medium to medium-dark tone and moderately strong saturation.

As learned from this post, certain phenomena gems can produce dramatic effects or colors. This factor contributes to gemstones’ desirability and value.

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