Every gemstone is special. But what makes some of them even more intriguing than the rest is the way they react with light to create unusual effects that are often known as optical phenomena.
Here are three of them that you should certainly watch out for…
1. Pleochroism
Pleochroism is the ability of a gemstone to display different colors or depths of hues when viewed from different angles or directions. Certain stones can showcase two different shades, while others can even go up to three.
This phenomenon is caused by differing absorption of light rays and only occurs in crystals with a high refractive index. It’s interesting to note that pleochroism can be enhanced through specific orientation in a gem, and in many cases, can be reduced or minimized through a well-executed cut.
Gemstones that can exhibit this phenomenon: Tourmaline, kunzite, unheated tanzanite, and emerald are known to display a high degree of pleochroism.
Aquamarine and peridot, on the other hand, showcase them in a rather subtle way.
2. Chatoyancy, a.k.a the Cat’s Eye
This striking optical phenomenon is caused due to parallel positioning of needle-like or fibrous inclusions within the gem. Certain stones, when cut precisely, reflect a narrow band of light or line, which we know as chatoyancy. The effect also closely resembles a real feline’s eye (hence the moniker). Faceted gemstones do not show this well, so they must be fashioned into a cabochon to best display the chatoyancy.
Sometimes, the cat’s eye effect can also exhibit a ‘milk and honey’ pattern. This happens when the stone displays two separate colors. While one side of the line is translucent, the other is milky. As light moves across the surface, the bands part and merge, and look similar to a cat’s blinking eye.
Gemstones that can exhibit this phenomenon: Chrysoberyl, tourmaline, aquamarine and moonstone.
3. Star Effect/Asterism
In this particular phenomenon, the gemstone exhibits a star pattern. The difference between asterism and chatoyancy, is that in the latter there is only one parallel needle inclusion. In asterism, on the other hand, there are three sets of rutile needles intersecting each other ‘three-dimensionally,’ to form a star.
Like the cat’s eye effect, asterism is also caused by light reflecting off parallel-aligned fibers and tiny needle-shaped inclusions. Gemstones that display a sharp and distinct star with straight, even rays intersecting at the top middle part of the stone are highly valued. It’s also interesting to note that four, six and twelve-ray patterns are quite common.
Gemstones that can exhibit this phenomenon: Garnet, spinel, sapphire, and ruby.
Other Honorable Mentions
In addition to the three phenomena mentioned above, certain gemstones also exhibit the following.
- Adularescence: Characterized by a bright, luminous or metallic reflection of light. It may also appear as a sheen that can be noticed when the stone is moved in certain directions.
- Aventurescence: This refers to numerous strongly colored or bright reflections of light that cause a spangled (or glittery) effect within the jewel.
- Play-of-Color: Displayed by certain types of opals, this phenomenon is characterized by weak to intense, regular or random flashes of spectral colors in response to light.
Also read – What Causes the Moonstone to Glow?
