Optical Effects of Phenomenal Cabochons…

Spectrolite, the trade name for labradorite from Finland, exhibits the phenomenon labradorescence − a broad flash of color across the surface caused by the internal structure of the stone. Similar to flashes of color in the Northern Lights, the blue and green hues in this spectrolite are fantastic. Left: labradorite from Madagascar. Center and right: spectrolite from Finland. Courtesy of Black Star Trading Company, Flagstaff, Arizona. Photo by Robert Weldon/GIA

The definition of a phenomenon as “an object or aspect known through the senses,” according to Webster’s dictionary, might explain why some gemstones are called phenomenal. Or it could be because these gems are “exceptional, unusual or abnormal.”

In reality, however, it is how the structure, inclusions and properties of these gemstones interact with light to return an unusual visual effect, or phenomenon, in the parlance of gemology, that make them phenomenal.

Regardless, we experience these gemstones as fascinating and recognize that they are, in fact, extraordinary.





There are several varieties of opal: black, white, crystal, jelly, boulder and boulder matrix, to name a few. Opals can be found in Queensland, New South Wales and South Australia. This black opal cabochon has excellent play-of-color, brightness and pattern. Courtesy of Lightning Ridge Opal, Glendora, California. Photo by Robert Weldon/GIA

Phenomena Caused by Structure

Opal, labradorite and moonstone form in microscopic layers of material that are successively stacked upon each other, giving rise to phenomena based on these unique structures. When light interacts with the different structures of gem material it causes flashes of color.

Let’s see how their different structures interact with light properties.

Opal is made up of millions of tiny silica spheres stacked in layers and held together by a mixture of slightly different silica and water. When light interacts with the silica spheres, a diffraction of light causes flashes of spectral colors. These colors depend on the size of the silica spheres. Smaller spheres produce blue to green spectral colors and larger spheres produce red. This phenomenon is called play-of-color.

Labradorite is formed by alternating layers of two feldspars that have different chemical compositions. When light interacts with labradorite, the diffraction of light causes the phenomenon known as labradorescence. The perceived colors are dependent on the thickness of the alternating layers and their refractive indices.


Moonstone is composed of two different types of feldspars that vary in thickness and regularity. In moonstone, the light is scattered in many directions giving it the characteristic cloudy bluish-white light, known as adularescence.

Iridescence happens when light passes through a thin, transparent film that has a different refractive index from the surrounding material. The color patterns in soap bubbles or on a thin film of oil on water are examples of iridescence. Pearl nacre and mother-of-pearl can have just the right structure to display iridescent optical effects. Orient is a special term used to describe pearl iridescence.



This chrysoberyl ring features a very fine cat’s-eye, which visually divides the cabochon in half and reflects a light and dark color called “milk and honey.” These attributes make the cat’s-eye chrysoberyl a collector gemstone. Courtesy Richard Krementz Gemstones. Photo by Robert Weldon/GIA

Phenomena Caused by Inclusions

A star or cat’s eye is the type of phenomena caused by inclusions of tiny bands of parallel needles or hollow tubes. When the bands of needles or hollow tubes are parallel, it’s called chatoyancy or cat’s eye. When they cross, a star is formed, called asterism. For best results in star and cat’s eye material, the base of the cabochon should be cut parallel to the inclusions to produce an even effect across the dome of the cabochon. Many gemstones possess the phenomena of chatoyancy and asterism: chrysoberyl, ruby, sapphire, spinel, garnet, tourmaline, diopside and quartz are examples.



Aventurescence is a type of phenomena caused by light reflecting from small, eye-visible, plate-like inclusions of copper or sometimes hematite inclusions in sunstone feldspar. (In sunstone feldspar the aventurescence is also known in the trade as schiller). In aventurine quartz, spangles of green mica produce the glittery effect and may also give the gem its perceived body color.


Color change is another type of phenomena that occurs most dramatically in alexandrite. Chromium causes alexandrite’s color, which accounts for a close balance between red and green, varying with the type of viewing light. The finest Russian alexandrites appear green in daylight and under fluorescent light. Under incandescent light, alexandrites appear red. This phenomenon coined the romantic phrase “emerald by day, ruby by night.” Although alexandrite, named after the Russian tsar Alexander II, is the most valued color change gem, certain sapphire, spinel and garnet varieties can also show color change.

Why Are Some Gems Fashioned as Cabochons?

The term cabochon comes from the French word “caboche,” meaning “head” and refers to a gemstone that has been shaped and polished as opposed to faceted. It is also common to carve softer stones as cabochons since minute scratches show much less on a cabochon than on a faceted stone.

Light properties and opacity − the ability of light to pass through the gem, are the main factors that come into play when deciding how to cut a gemstone. If a gemstone is opaque it is usually fashioned as a cabochon. Stones with asterism and chatoyancy are cut as domed cabochons to best portray the fine, needle-like inclusions that produce these two phenomena, which would otherwise not be visible in a faceted cut.

The final decision ultimately falls on the lapidary artist or the client’s preference, but they are often fashioned into cabochons to show them off to their best − phenomenal − advantage.

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**Alexandrite also considered to be June’s birthstone.

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Imperial Topaz

Imperial Topaz

There is so much bad information out among the public regarding topaz that it is hard to know where to begin.  The need for a birthstone with Fall colors is undoubtedly why topaz was chosen for November.  Well, it is the best of choices for color and beauty but a less fortunate choice for availability.  True topaz in the color spectrum we all associate with Fall currently comes only from a small mining district in Brazil.  The availability of the material is comparatively low and the price is comparatively high.  In the USA most people are completely unaware of true Imperial Topaz—the legitimate birthstone for November.

Imperial Topaz got its name from a deposit of gems found in Imperial Russia.  It has been said that many of these gems found their way to the royalty of Russia and got the sobriquet “Imperial Topaz” from that association.  Whether from a lack of quantity at the mine or from the political issues following the 1917 Russian revolution gems from the Russian source have not been generally available for decades.

In the 1700’s a good find of Imperial type topaz occurred in Villa Rica, Brazil.  Various mines in and around the region have been discovered and mined since that time.  Many mines have been discovered and played out with very few mines producing gems on a regular or commercially viable basis.

My good fortune was to begin traveling and buying gems at the source quite early in my career.  In addition starting out as a gem cutter helped me know gems more intimately than people who had been in the business decades longer than I had.  Traveling to Brazil in 1978 I was lucky enough to find big productions of many gems including Imperial Topaz.  As it turns out buying Imperial Topaz is a complicated issue but good introductions from a Brazilian friend along with a lot of gem knowledge made for comparatively smooth sailing.  Big wholesale accounts bought large quantities of Imperial topaz from me right away since I brought in higher quality gems than other wholesalers brought to the party at that time.

So, what is it that people have been told and what is it that people have been sold? Years ago, here in the USA, a lot of smoky quartz was sold and called smoky topaz and sold as such.   Smoky quartz is a logical (somewhat) substitution for true topaz.  It has Fall colors, is inexpensive by comparison, and is readily available.  It has taken most of my 38 years in the business, however, for the true students of gemology to eradicate the misnomer: Smoky Topaz.

Another member of the quartz group of gems has also been sold as a replacement for Imperial Topaz.  That gem is Citrine.  Citrine is the color variant of quartz which covers a spectrum from pale straw yellow, yellow, yellow orange, orangery gold, orangery red and even a fairly pure red.   Most of the citrine that makes it to the market in the USA is in the middle of this spectrum.  Quartz that finds its way into citrine for the jewelry market currently comes mostly from Brazil but also recently from Africa.  From time to time Citrine has also been mislabeled as being Topaz.

Another source of confusion regarding topaz is the arrival of blue topaz into the marketplace beginning sometime in the 1970’s.  Naturally colored blue topaz has been noted as coming from many locations around the world.  As it turns out the source of the color in naturally colored blue topaz is naturally occurring sources of radiation in the earth.  We all experience every day what is known as background radiation.  Well as it turns out otherwise colorless topaz can become blue due to eons of exposure to natural background radiation. Topaz from some areas seems to be more susceptible to the radiation and is more likely to turn blue.  When I was first in the business I cut some blue topaz that was from Texas.  Also at that time there were notable mineral specimens from Rhodesia (Currently Zimbabwe) which were the most vibrant blue yet seen.  The color was natural the stones came out of the earth naturally sky blue.

Close to the same time I was cutting blue topaz from natural sources there were people experimenting with coloring of topaz with man-made sources of radiation.  The man-made sources of radiation had much more energy in a short period of time than the natural sources and the resulting gems were a much more marked blue than the naturally colored stones—and there was a virtually unlimited supply of the source material—colorless topaz.  Well the beautiful color of the newly available gem made it an instant hit with the public all over the world but also added confusion to the November birthstone.  People who wanted a topaz birthstone but who also preferred blue to the fall colors had a new choice.  Jewelers who wanted a new affordable topaz for their customers had something new to offer the November babies–all good developments but also confusing to the consumer from time to time.  Just so you know, the US government has promulgated strict guidelines ensuring the safety of blue topaz to protect the consumer.

All of the gems that one way or another may be used as the November birthstone are beautiful and have justification for use as the November birthstone.  Simply enjoy whichever stone you buy knowing what it is.

written by John Ramsey, master gem cutter