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 the value of gemstones are affected by these 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 try to answer in today’s blog post.

What causes phenomena in gemstones?

Phenomena gems are caused by 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 how rare and desirable they are to the consumers.

Top grade star corundum (rubies or blue sapphires), opals, alexandrite and cat’s eye chrysoberyl are considered valuable phenomena gemstones. Moonstones and labradorite are desirable but a lot more abundant and therefore less valuable. Others like aventurine quartz are plentiful and inexpensive.

A. PHENOMENA CAUSED BY STRUCTURE

Gemstones like moonstone, labradorite, opal, and some pearls owe their phenomena color to the way each gem’s structure affects light. All of these gems are made of small, minute, repeated layers.  When the light rays enter these layers, they are ‘reflected or refracted’ 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 perfectly parallel or synchronized – each peaks and troughs corresponding exactly with other’s peaks and troughs – they reinforce each other, which make brighter colors.  If the light waves are exactly opposite to each other, with each peak of one wave occurring in exactly the opposite trough of the other wave, they cancel each other, resulting in darkness.

PLAY-OF-COLOR

Diffraction is a special type of interference phenomenon.  Instead of producing areas of only bright and dark, this phenomenon produces pure spectral hues red, yellow, orange, green, blue, and violet.   This happens with gemstones that have different stack layers of alternating materials 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 it made up of millions of tiny spheres (called 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 is called play-of-color.  Smaller spheres produce blue to green play-of-color and larger spheres produce red.

Coober_Pedy_Opal_2
Multi-color rough crystal opal from Coober Pedy, South Australia, showing almost all color spectrum (blue, green, red, yellow) © Wiki Dpulitzer

Opal is actually quite unique as they come in all different quality grades and therefore different ‘price levels’ in the gemstone market. Opal is definitely considered one of the most popular phenomena gems and it is mounted in jewelry to suit any consumer’s budget.

To read more about opals I recommend reading this blog article:  Natural Opal buying guide – How to check and understand difference between different opals?

IRIDESCENCE

Iridescence is one of the most common interference phenomenon and it happens when light passes through a thin, transparent film that has different RI from the surrounding material.    Examples of this effect can be 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 which penetrate through the film reflects from the bottom.   Depending on RI and thickness of the film the waves are either travelling together or out-of-phase wavelengths, so as you learned earlier the color can be brighter or darker.

Most famous gemstones who have this effect is the Pearl Nacre also known as the mother of pearl as it is composed of two different materials: aragonite and conchiolin. In the trade, a special term ‘orient’ is used for pearl iridescence effect.

Mother-of-Pearl.jpg
Mother of Pearls lot, which my grandfather acquired in 1960s.  Though not 100% clear in this image these stones does display “iridescence” phenomena

ADULARESCENCE

This phenomenon is something we see naturally every day in the morning, the beautiful “blue sky.”   The blue color is caused by 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 blue sheen when light reflects its internal structure.  Moonstone’s structure isn’t regular and actually made up of alternating layers of two different feldspars and the layers vary in thickness and regularity.  These variations cause the incoming light waves to scatter and therefore makes ‘blue sheen.’

B. PHENOMENA CAUSED BY INCLUSIONS

In general, inclusions usually take away from a gem’s beauty and the more inclusions there are, the lower is the value of a gemstone.  However there are some inclusions of ‘the right type’ and ‘the right position’ which can cause spectacular optical effects.  These inclusions can produce by reflection of the light rays, a ‘cat’s eye’, a ‘star’ or provide glittery sparkle.

CHATOYANCY

The most famous ‘chatoyancy’ gemstone is cat’s eye chrysoberyl.  The gem’s striking “eye” is caused by parallel needle-like inclusions or fine hollow tubes which when rough cut correctly reflect 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 gems especially chrysoberyl show ‘milk and honey’ look.  This affect is seen if the light source is placed at the right angles of the cat’s eye so one side of the stone looks milky while the other retains the honey look.

Other gemstones like Tourmaline or quartz can also chatoyancy.  In fact, I just discovered that even tanzanite if having the right type of inclusions can also display cat’s eye, this is exceptionally rare and I was quite shocked with 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

The value of cat’s-eye chrysoberyl is directly related to factors like quality of its phenomenon, its body color, transparency, and 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.
4.5mm-Alexendrite-Cats-Eye-Parcel
4.5 mm cat’s-eye chrysoberyl which I game across in JTC Bangkok.  These are consider 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.

ASTERISM

Asterism occurs when corundum crystal (both rubies and sapphires) have three sets of rutile needles intersecting each other ‘three dimensionally’ to form a six-rayed star.   This phenomenon is very similar to how chatoyancy is formed, though the difference is that in 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.  However, star sapphires or star rubies are considered the most valuable.  The best star corundum is semi-transparent which are very rare.   Most star rubies and sapphires are translucent or opaque.  Star corundum value is judged by the color of the gemstone and by ‘how sharp and where the position of the star’ is in the gemstone.

To show the 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 portion of the stone.

How to Evaluate Star Corundum Value

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 spot light because diffused fluorescent lights won’t show the star. Then while pointing the light source you should 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

When a cabochon sparkles with thousands of tiny reflections as it moves in the light (or simply a glittery effect) this phenomenon is called aventurescence.   This phenomenon is caused by light reflecting from many small, but eye-visible, plate-like inclusions in the stone.  When the stone is rotated in the light these inclusions reflects the light back to the viewer.

An example of aventurescence gemstone is sunstone feldspar, which has copper or sometimes hematite inclusions which causes the aventurescence and 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.

COLOR CHANGE

The most famous color change gemstone which buyers are dying to acquire is alexandrite chrysoberyl and top grade ones are usually featured in exclusive auction catalogs.  The reason why they are most sought after is because of the romantic history of Russian czars prizing it as well as its fascinating color change phenomenon.

In alexandrite the crystal has trace element chromium which features in both ruby and emeralds and that is why they can portray both colors.  Though the difference is that alexandrite allows almost equal amounts of red and green light to pass through it, but absorbs light most strongly in the middle of the visible spectrum.

Because the gem’s absorption is closely balanced between both red and green, its color varies based on the type of lighting you view the gemstone under.  When an 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.

However 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.   Although alexandrite is the most valued color-change gem, some others, like sapphire, spinel, and garnet, can also show this effect.

How to determine value of alexandrite?

2.10 carat Alexandrite
2.10 carat Alexandrite from Orissa, India. Stones from this origin usually doesn’t have strong color change when compare to finest Russian options.

Alexandrite hues range from very strongly bluish green through yellowish green in daylight and orangey red through purple-red in incandescent light.

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

Finest-color alexandrite is green to bluish green in daylight and red to purplish red in incandescent light with medium to medium-dark tone and moderately strong saturation.

As you have learned from this blog post, certain gems when they interact with visible or different light sources can produce different color or striking phenomena.   When we include this factor with the other six factors which I have discussed in the past – color, carat weight, clarity, cut, origin & treatment – we can see how it can dramatically affect the gemstone desirability and value.

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