By Lauriane Lognay
What is an optical phenomenon? Plainly put, this term embodies the result of the way light interacts with the crystalline structure, inclusions, and/or internal structure of a gemstone. It is, in essence, the reflection, diffraction, absorption, and/or diffusion of that light. Without even knowing, we encounter these phenomena on a daily basis, whether in opal, moonstone, alexandrite, or cat’s eye chrysoberyl—that said, one should be careful not to confuse optical property, such as dispersion in a diamond, with optical phenomena.
This article will explore optical phenomena in gemmology, offering an explanation as to why and how it occurs, expanding this knowledge, and helping jewellers better explain the wonders of gemstones to their clients.
Pleochroism refers to seeing different colours in a gemstone depending on the direction of observation or viewing. Some gemstones have two colours visible at a time (dichroism), while others have three (trichroism).
Some examples of dichroism include:
- amethyst (grey purple/reddish or bluish purple);
- tourmaline (two tones of the same colour);
- ruby (red/orange red);
- aquamarine (colourless or sky blue/blue or green);
- emerald (blueish green/yellowish green);
- heated tanzanite (blue/violet); and
- peridot (green/yellowish green).
Meanwhile, examples of trichroism include:
- andalousite (greenish/yellowish/reddish brown);
- iolite (violet blue/colourless/yellow);
- unheated tanzanite (violet/blue/yellow or fuchsia); and
- kyanite (dark blue/violet blue/pale blue or colourless).
Pleochroism is mainly attributed to the differential selective absorption of wave lengths. The aforementioned is the result of the light’s polarization in two vibrant perpendicular rays. Gemstones that are isotropic (e.g. spinel, garnet, diamond, etc.) are not able to have any pleochroism; only the anisotropic gemstones (e.g. sapphire, ruby, beryl, etc.) can have the optical phenomenon.
Iridescence, adularescence, and labradorescence
In gemmology, iridescence, adularescence, and labradorescence are often grouped together, but each offers its own unique beauty. Iridescence, for instance, is a lot like adularescence; however, while the latter primarily offers a blue lustre, iridescence can show all colours of the rainbow in the reflection observed inside a gemstone. The colourful sheen can also be seen shifting, depending on the incident light change and how a gemstone is moved. Some industry professionals consider it the same iridescence you can also observe in a fissure or a broken stone, while others do not consider this an optical phenomenon. Popular examples of iridescence in gemstones include rainbow garnet (mostly from Japan), fire agate, pearl, nacre, iris agate, and rainbow obsidian.
Adularescence, seen in moonstone, is blue or white reflection observed inside a gemstone. Like iridescence, this blue can be seen moving, depending on the incident light change. The light is diffused inside and creates the blue sheen.
Like iridescence and adularescence, labradorescence (observed in Labradorite) is also the reflection and interference of light inside a gemstone. Simply put, because Labradorite and moonstone are from the same family (called feldspar), the gemmology community decided to name the optical effect differently for the two gems.
More often than not, the iridescence, adularescence, and/or labradorescence observed in a gemstone is caused by the reflection and interference of light on layers inside the stone. For example, in Labradorite, the labradorescence is caused by the overlapping thin layers of calcium and sodium, while fire agate contains thin layers of iron oxide inside a quartz. Nacre and pearl are each composed of layers upon layers of conchiolin and aragonite. Though it is iridescence, many in the pearl business more often than not call the effect ‘the Orient.’ Iris quartz is one of the few gemstones in which the iridescence is entirely caused by fissures inside.
Chatoyancy (Cat’s eye)
Chatoyancy is the appearance of a luminous band, generally white, which slides on the surface of a cabochon under a light and creates an effect reminiscent of the slit of a cat’s eye. For many, a gem’s desirability is largely dependant on it’s banding (the thinner and clearer this effect, the rarer and more sought-after the gem). While this banding is present in many stones, chrysoberyl is the only variety that can technically be called a ‘cat’s eye’ without referring to the gemstone by name. Other examples of gems with this effect include tourmaline, apatite, obsidian, quartz, opal, tiger eye, diopside, beryl, scapolite, and moonstone.
In most cases, the cause of the chatoyancy has to do with the inclusions inside the stone; it is the reflection of the light on numerous acicular inclusions oriented in the same direction. This can be needle inclusions or sometimes, like in aquamarine, parallel tubes (as long as there are a lot of them, all facing in the same direction). In these cases, the banding will appear generally perpendicular to the mentioned inclusions.
Asterism is the appearance of a star-like structure that, similar to the chatoyance effect, appear to slide on the surface of the cabochon under a light. These stars can have four, six, or even 12 branches to them. The clearer and better represented a star shape is, the rarer and more sought after the gemstone is considered. Gemmologists often use the angle of the star on the stone to help determine its identity. While not diagnostic, this can give a good idea of what it is not and, in turn, what it probably is.
There are two types of asterism: epiasterism (in reflected light) and diasterism (in transmitted light). The former is the variety seen on the market. You cannot really see it well unless you have a light on it (for the epiasterism) or under it (for the diasterism).
This effect can be seen in several gemstones, including:
- Corundum (sapphire, ruby), which can have six to 12 branches caused by rutile needle inclusions. The angle of the asterism is 120/60 degrees.
- Spinel, which is similar to corundum, but can have four to six branches only (with 120/60 degree for six branches or 90/90 degree the four). The needle inclusions are generally sphene.
- Diopside, which can have four branches caused by magnetite needles inside the stone.
- Quartz, which has six branches (as well as a rarely found diasterism-type in pink quartz).
- Garnets, which can have four, six, or 12 branches caused by rutile inclusions. Generally, the angles are 110/70 degree for the four branches. Almandine garnet can also be found with diasterism (though this is rare).
- Moonstone, which has peculiar elongated hematite inclusions that can cause an asterism to form. It can show four to six branches.
As mentioned, asterism is caused by the reflection of light on numerous tube- or needle-like inclusions within a stone. These inclusions generally go in two or three directions, creating a number of branches on the star. Natural star gemstones will be cut in fat cabochons; the bottom of the stone will not be flat, but bombe (often times not even polished). This keeps as many inclusions as possible inside the stone, which helps form a strong star. Synthetics, however, are generally completely flat under the cabochon dome with a visible star even with no light source.
This effect refers to gemstone ‘spangles,’ for lack of better term. When the light shines on a stone’s inclusions, it is reflected, creating a sort of glitter effect inside the stone. To form a real impact on the appearance of the stone (and to truly be called aventurescence), these inclusions have to be countless.
The strongest example of aventurescence can be observed in sunstone, where the effect is caused by copper and hematite platelets from within. The next best example would be the aventurine quartz, which is generally a colourless stone with mica platelets inside that give it a green appearance. Obsidian, with its bubbles and crystals inside, is another example, as is bloodshot iolite. This is a normal iolite, but with red hematite platelets oriented in the stone, which give it a ‘bloodshot’ appearance (hence, the name).
Additionally, goldstone serves as a popular imitation of sunstone. This can be found in blue and gold and is a prime example of aventurescence imitation.
Colour change is the phenomenon in which a gemstone will change colour depending on the light source. Light sources used to determine if a gemstone is truly colour change or not include incandescent (white) and daylight (yellowish). The change in colour must be obvious; not just a shift in tone as seen in tourmalines and other similar gemstones. The most popular gemstone where colour change is observed is the alexandrite, including natural mined from Russia (which goes from raspberry red to bluish green) and synthetic (which goes from purple to bluish green).
Other examples of the optical phenomenon include:
- natural corundum (blue to purple);
- synthetic corundum (pinkish purple to grey violet);
- garnet (purple to greenish blue);
- Csarite (pink to green);
- glass and cubic zirconium (purple to blue—wide variety overall, from colourless to pink, to yellow, to green);
- spinel (purple to blue); and
- fluorite (purple to blue).
What causes the colour change is fairly complicated. Overall, the change is due to the spectre of absorption of the light going through the gemstone, combined with the colour of the type of light source (incandescent light is rich in red, while daylight is rich in blue).
Play of colour
Play of colour refers to a multitude of spots and/or dots of spectral hues, which change and appear to be in motion in a gemstone when moved. Similar to colour change, this effect also depends on lighting and the environment of the stone. These colours include red, blue, green, yellow, and so forth.
The perfect examples of gemstones with this effect are natural opal (sourced from Mexico, Australia, and Ethiopia, mainly) and synthetic opal.
These spectral colours are the result of a diffraction of the light (whereas iridescence is the reflection of the light on the layers and/or inclusions). Opals are composed of microscopic spherules; when these are numerous enough in a stone and placed in an orderly manner, light passes through the gaps between them and is diffracted. Larger gaps produce red to yellow/orange colour, while smaller ones create blue to green.
While opalescence in a stone is not often discussed, this effect is more common than we think (and also one of the simplest). This is when a gemstone has a milk-like appearance.
This milky effect is most often the result of tiny but visible inclusions/particles within a stone. It can be present in practically any gem.
In gemstones, the Usambara effect is among the rarest optical phenomena. While the concept is not often studied and much is still unknown, it is certainly worth mentioning.
It was once believed the Usambara effect was only present in tourmalines recovered from the Umba Valley in Tanzania—nowhere else. With this effect, one can observe a stone change from green to red in the path of a light shined directly through it. It is the ability of the material to change colour in relation to the light travelling through it.
The Usambara is considered different from any colour change effect previously mentioned. Changing the environment does not create it, changing the light source over it does not create it, and the reflection of the light on it does not create it; indeed, it is only seen and observed in the path of the light passing through the stone.
It was later discovered, along with these specific tourmalines, a few garnets, corundums, epidote, and kornerupine can also have the effect.
Like the Usambara effect, tenebrescence is an unusual optical phenomenon and not often talked about.
A colour change can be observed in the stone after exposure to ultraviolet (UV) light. Unlike regular colour change, the reaction is reversible and, therefore, not stable in the stone. It is a reversible change of colour upon exposure to light, almost exactly like progressive shades/glasses. When the stone gets heated, however, it loses the ability to change colour in sunlight.
The phenomenon was observed mainly in a gemstone called hackmanite (a variety of sodalite), which can go from colourless to dark purple or pinkish upon exposure to sunlight. Some of the largest quantities were found in Mont Saint-Hilaire, Qué. (O Canada!). The stone, when taken out of the sunlight, will gradually get its original colour back. The effect was also observed in a few zircons, spodumene, tugtupite, and some scapolite.
Let your heart decide
With all these wonderful and intriguing optical occurrences in gemstones, one would be hard-pressed to choose a favourite. It’s incredible to think mixing a few chemicals together with just enough pressure and time can create gemstones—which, in turn, would then create phenomena the human eye is able to see and appreciate. The numerous plays of colour in opals serves as just one fine example of nature’s greatest gift. I think we have yet to discover the best in the gemmology world.
Lauriane Lognay is a fellow of the Gemmological Association of Great Britain (FGA), and has won several awards. She is a gemstone dealer working with jewellers to help them decide on the best stones for their designs. Lognay is the owner of Rippana Inc., a Montréal-based company working internationally in coloured gemstone, lapidary, and jewellery services. She can be reached via email at firstname.lastname@example.org.