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Chameleon, I Am


Color-Change Alexandrite - Emerald By Day & Ruby By Night

Another type of Phenomenal Gems is the Color-Change Gemstone. These are gemstones that display different colors according to changes in lighting condition, much as a Chameleon monitor lizard, which changes the color of its scale to camouflage itself against other predators to enable it to blend in to the environment.

Chameleon Lizard Changes Its Scale According To The Environment

Now the chameleon does not have the innate ability to change colors according to it’s whims and fancies. External factors such as temperature, danger and emotions of the chameleon are responsible for this. It has a major distribution of melanophores in it’s body which are responsible for the change in color. Since the chameleon’s temperature is controlled from outside its body, its reaction to hot and cold temperatures, leads to a change in the skin color. In addition fear, territorial instincts and danger may make them change their color. All of this is involuntary.

A Vivid Color-Change Sapphire

The term Color-Change in gemology is generally used for gemstones that exhibit a distinct change of color hue when the gemstone is exposed to different lighting illumination.  Thus, an Alexandrite may appear green under the cool natural daylight (or fluorescent lighting) while under incandescent light (eg halogen lighting, yellow torch light or chandelier lighting) it appears red.  This type of Alexandrite is adroitly known as Emerald By Day & Ruby By Night.

A Beautiful Vivid Color-Change Alexandrite

The cause of its Color-Change nature is due to the selective absorption of spectral colors within the stone.  These Color-Change gemstones have two approximately equal sized transmission windows.  A red gemstone appears red because it absorbs all frequencies of light except red. A gemstone that absorbs all frequencies except for blue and red light will appear blue when the light is rich in blue wavelengths (e.g., fluorescent light) and red when the light is rich in red wavelengths (e.g., incandescent lighting).

An Oval Cut Color-Change Garnet

Apart from Alexandrite other gemstones that may exhibit Color-Change phenomenal includes spinel, garnet and corundum (rubies and sapphires).

A Pear Faceted Cut Color-Change Spinel

Color-Change gemstones are a collector’s item and can be expensive.

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You may have missed other posts of interest. To read them, please click on the picture

Market For Natural & Synthetic Gemstones


Boule Is A Synthetic Corundum

Hi A Arthur

I read with interest on your post on Natural, Synthetics and Stimulants.  To be honest, I am only starting to learn about gemology.  I found your posts interesting as there are many pictures and the explanations are very clear.

I have a few questions.  With so many synthesis of gemstones, would synthetic gemstones affect the market of the real natural gemstones.  Like HPHT diamond, would these diamonds affect the market for natural diamonds.  Are there a real market for synthetics since some of these manufacturing processes are very costly?

Thank you

Jim Henry, London

Hello Jim

Thank you for your enquiry and I am glad you found my posts useful.

Synthetic gemstones play an important role in the jewelry industry as well as in many manufacturing industries because of their properties.

The sapphire glass top of watches are synthetic corundum.  Synthetic star rubies and faceted rubies are used for costume jewelries (or they may pass off as naturals), while HPHT diamonds also have ready buyers who would like to have a ‘biggie’ diamond but cannot afford the price.

A Synthetic Corundum Manufacturing Process

Corundum (read here) is used in many manufacturing processes as abrasives, for sandblasting as well as in refractories. The pointed war-head tip of missiles are coated with corundum because of their hardness as they can withstand the friction and abrasion of atmospheric particles.  Synthetic diamonds are also used as abrasives, dressing, machine and cutting tools, bore drills in the oil industry as well as widely used in the semi-conductor industry.  Synthetic quartz has a piezoelectric effect and they are used as pressure gauges, oscillators, resonators wave stabilizers, precision instruments and other electronic devises.  They are also used as heat-ray lamps, prism, and spectrographic lenses.

In the early 70s when the synthetic market was growing at a hectic pace, interest in natural gemstones increased by leaps and bounds too.  People are aware that natural gemstones are a depleting resource and if naturals and synthetics can be identified and differentiated then the two markets can develop side by side without much conflict.  In fact, synthetics have sparked consumer demands and help drive up the price for naturals.

Of course one must be aware that the jewelry industry has also its fair share of scams, con-jobs and people trying to make a fast buck by selling synthetics as naturals.  Some unscrupulous sellers may knowingly sell synthetics as naturals.  However, the biggest pitfall is misidentification as both synthetic and natural gemstones are made of the same composition with similar physical and optical properties and only a trained gemologist can identify them.  There are always tell-tale signs between a natural and a synthetic.

Hence, if you are buying any jewelry go to the jewelry store whom you trust. You may have missed other posts of interest. To read them, please click on the picture

Natural, Synthetic & Stimulant Gemstones


As discussed in the previous post (Colored Stones, Precious & Semi-Precious Gemstones, read here) for the initiated one has to understand clearly what is a natural, synthetic and stimulant gemstones.  This post may take you less than 3 minutes to read but once you have digested the material it will stay with you forever with a clear understanding on the differentiation of the 3 definitions.

A Natural Gemstone is formed by Mother Nature without human help.  Most of these are minerals produced by Earth’s rock forming processes and may take millions of years for its formation.  Others may be of an ‘animal’ nature like natural or cultured pearls or corals.  Yet there is also another gemstones class formed by plants, example amber, which are formed by the hardening of the plant resins.

Synthetic Gemstones have the same physical, chemical and optical properties as those of its natural counterparts, except that Synthetic Gemstones are produced by a technological process as opposed to Natural Gemstones.

Synthetic diamonds are produced by a few processes notably HPHT (High Pressure High Temperature) or the CVD (Chemical Vapor Deposition).

Synthetic rubies and sapphires are quite commonly produced under lab-controlled processes.  These gemstones are called lab-grown or lab-created gemstones.  Other popular synthetics are synthetic emerald, synthetic quartz and synthetic spinel.

A stimulant is an imitation that is almost a look-alike to the natural gemstones.  Glass and plastic are the most popular material for making imitations.  Hence, a calcite dyed green bangle is often imitated as a jade bangle or a green quartz as a jade cabochon.

So it only takes you 3 minutes to digest this post but to those who are previously not clear about these 3 definitions now you will have a comfortable understanding of natural, synthetic or imitation gemstones, instead of lumping those synthetics or imitations as faked gemstones.,,,, You may have missed other posts of interest. To read them, please click on the picture

Toughness of a Gemstone


Usually when gemstones are defined as durable, they include two distinct traits, hardness and toughness.  These terms are not the same and people are often confused between these two.

Cleaving A Diamond

A mineral’s hardness is its ability to resist scratches or abrasions, and hardness is defined on the Moh’s Hardness Scale from 1 to 10, where diamond is the hardest of all mineral with a 10 on the extreme end.  Remember that the Moh’s Hardness Scale is a relative scale and not an absolute scale.

A Chinese Artisan Carving A Jadeite Jade Block

A mineral’s toughness is its ability to resist being fractured. Toughness means resistance to breaking, chipping or cracking — in general, how well a stone holds up under mechanical stress, such as impact from a fall. Toughness rating is usually stated as Exceptional, Excellent, Good, Fair, or Poor.

Even though diamond is the hardest mineral it can chip and fracture in normal day to day wear and tear.  Diamond is formed in the cubic crystal system and has 4 perfect cleavage direction.  A cleavage direction is the weakness plane in the molecular crystal of a diamond.  Hence, an experience diamond cutter will look for the cleavage plane to cleave a big diamond. The toughness rating of diamond is only ‘good’ while that of Hematite is ‘excellent’ while its hardness is only 5.5.

Jadeite Jade and Nephrite Jade have a toughness graded as Exceptional, these two minerals are one of the toughest in the mineral kingdom.  Hence, these minerals can be carved into minute details without fracturing the stone.  Jadeite Jade is composed of micro-crystalline crystals that interlock with each other, making the mineral exceptionally tough.  Nephrite Jade is of a fibrous nature consisting of extremely dense and compact fine-grained fibrous tremolite and actinolite mineral.  Hence, jade can be sawed and carved into very delicate and extremely durable objects of art and adornment.,,

The All Seeing Eye

Chrysoberyl Milk & Honey Cat's-Eye

Chatoyancy, is an optical reflectance effect seen in certain gemstones. Coined from the French word “œil de chat,” meaning “cat’s eye,” chatoyancy arises either from the fibrous structure of a material or from fibrous inclusions or cavities within the stone.

A lot of people have mistaken that cat’s eye is a type of gemstone with a band of concentrated yellow or whitish light across the stone, like the slit of the eyes of a cat. Now it must be stated clearly that the cat’s eye effect is a light phenomena and it occurs in a variety of gemstones.

When light strikes the surface of a gemstone some light will pass through it while some will be reflected back to the eye.  When reflection occurs on the surface of a gemstone the amount and quality of light is called luster.

However, there may be a variety of gemstones where light reflection occurs slightly below the surface. This reflection is called sheen and the most striking example are those gemstones which produce the cat’s eye effect.

A Bluish Chrysoberyl Cat's Eye

Normally cat’s eye gemstones have dense fine needle-like inclusions or tiny hollow tubes running parallel to each other and parallel to the plane of the base of the gemstone.  Most gemstones which exhibit the chatoyancy effect are cut into cabochons to produce this phenomena, where the sheen is concentrated across the dome of the stone into an eye.

The Slit Of An Eye of A Cat and A Spool Of Thread With A Concentrated Light Source

A good everyday example would be the light reflected from a spool of thread.  Using a strong fiber optic light I beamed it at a spool of fishing thread and took a picture.  Here, the cat’s eye effect is produced as shown in the picture.

Tourmaline & Moonstone Cat's-Eye

The best known cat’s eye gemstone is cat’s-eye chrysoberyl, where an excellent stone can be more costly than a well cut diamond of similar weight.  Top grade cat’s-eye is semi-transparent with a slightly greenish yellow body color and should be free from eye visible inclusions.  The ‘eye’ should be silvery or yellow in color.  When the ‘eye’ moves according to the light source, it should be narrow, bright, sharp and concentrated without any breaks or waviness.

Scapolite and Quartz Cat's-Eye

Among the gemstones that exhibit the cat’s-eye effect are Alexandrite, Moonstone, Quartz, Tourmaline and Scapolite.  Most of these gemstones must be cut into cabochons to exhibit the cat’s-eye effect.  However, Tiger’s-eye Quartz is one of the few gemstones that shows strong chatoyancy even when cut into facets or flat surfaces.

Tiger's Eye Quartz


A Star Is Born


A Star Ruby With Excellent 6-Ray Star At Its Dome

Asterism is the effect of a six-ray or four-ray star on top of a dome of a cabochon gemstones.

A Beautiful 6-Ray Star Sapphire Set With Ascending Diamonds

Fine Silk Inclusion Of A Star Ruby Intersecting At 60 degree

In effect, asterism is the same as chatoyancy (Read here for Phenomenon Gems), except that there are 3 sets of fine parallel fibers lying along the same lateral axes and intersecting at 60 degrees to each other.

In the Burmese rubies these fine parallel fibers are the silky inclusions which fluorescence crimson red under UV light.

Prominent Silk Inclusion In A Burmese Sapphire

The most prominent of the 6-Ray Star effect is found in Corundum (Read here for Corundum).  The beautiful star in Rubies and Sapphires are much sought after by gems and jewelry connoisseurs.

A Ruby In The Rough, Notice The Hexagonal Shape

I have cut some star rubies in Myanmar before being taught by a gemstones cutter.  We used some oil to rub on the surface of a rough ruby.  The stone is to be rotated until you find a fuzzy star on one of the rough surface.  This should be the apex of the dome of the star.  It is easy to find it but to really cut out an excellent star effect from a corundum you need to have varied experiences for the rough is very expensive.

The 4-Ray Star Diopside

Star Diopside, a black gemstone, is relatively unknown to the public, has a unique 4-Ray star which is created by platelets of magnetite instead of parallel intersecting needles.

The 6-Ray Star Rose Quartz

Rose quartz which crystallizes in the hexagonal system also has a 6-ray Star effect.  However, asterism in Rose Quartz is visible in transmitted light rather than reflected light.

There will be another post later on how to evaluate the quality of a star on a ruby/sapphire.

Sources & Citations,,,,,


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