Phenomena in Gemstones – Impact On Value December 26, 2022 – Posted in: Gemstones – Tags: Gemstone, Gemstone Phenomena
Have you ever heard of “Gemstone Phenomena”? If you have some knowledge regarding the gemological world, then you’ve probably heard of precious stones with unique optical properties. These unique optical properties are referred to as “phenomena” in the gemstone industry. For those unfamiliar, phenomena are rare or exceptional optical effects that occur due to a gemstone’s changing illumination conditions. So, what interesting optical qualities are necessary to discuss?
It is essentially the light’s reflection, diffraction, absorption, and/or diffusion. We encounter these phenomena daily without even realizing it, whether in opal, moonstone, alexandrite, or cat’s eye chrysoberyl. However, one must be careful not to confuse spectral properties, such as diffusion in a diamond, with optical phenomena.
Asterism is the appearance of a star-like framework that appears to slide across the surface of a cabochon when illuminated, similar to the chatoyance effect. These stars could have four, six, or even twelve sections. The more distinct and well-represented a star shape is, the more uncommon and valuable the gemstone. Gemologists frequently use the angular position of the star on the stone to help us identify it. Although not diagnostic, it can provide a clear picture regarding what it is not and, consequently, what it is most likely.
Asterism is classified into two types:
- Epiasterism (in reflected light)
- Diasterism (in transmitted light)
The former is the most common type found on the market. You can’t see it properly unless you shine a light on it (for the epiasterism) or beneath it (for the diasterism). Asterism is prompted by the reflection of light on a stone’s multiple tubes or needle-like granules. These inclusions typically travel in two or three directions, resulting in a plethora of divisions on the asteroid. Natural star gemstones will be cut in fat cabochons, with a bombed bottom rather than a flat bottom (oftentimes not even polished). This tends to keep as many inclusions as possible within the stone, assisting in the formation of a strong star. Synthetic materials, on the other hand, are generally perfectly flat under the cabochon dome, with a visible star even in the absence of a light source.
Chatoyancy or Cat’s Eye Effect
Chatoyancy is just another optical effect that some gemstones may display. A well-cut chatoyant gemstone will typically display a distinct firm line of reflected light caused by fibrous inclusions within the gem. It’s beneficial to understand that when it’s cut as a cabochon, you have a good opportunity of witnessing this effect. For those who might not know, the word chatoyant is derived from a French term that means “eye of a cat”. If you have a cat, you will understand when we talk about the distinctive cleft that runs through the center of the eye. This is where the phrase “cat’s eye” is used. The mineral Chrysoberyl is well-known for exhibiting this effect.
Since opalescence in a stone is underexplored, it is more prevalent than we realize (and also one of the simplest). This gemstone has a milk-like appearance. The milky effect is usually caused by relatively small but noticeable inclusions/particles within a stone. It can be found in almost any gem.
The phenomenon of color change occurs when the color of a gemstone changes depending on the light source. Incandescent (white) and daylight illumination are used to determine whether a gemstone is truly colour change or not (yellowish). The colour change must be noticeable, rather than just a tone shift as found in precious gemstones and others. The most common gemstone with colour change is alexandrite, which comes in both naturally mined forms from Russia (which change from raspberry red to bluish green) and synthetic forms (which go from purple to bluish green).
Cat’s Eyes Open and Close
It’s yet another effect that is common in Chatoyant materials. When you shine a bright light through the edge of the stone and spin it by 45 degrees, you can see the opening and closing effects. The standout feature of this effect is that whenever the stone is twisted 45 degrees, the cat’s eye opens and closes.