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METALSMITH'S GUIDE TO GEMSTONE LIGHT PLAY TERMINOLOGY

Light is the essence of gemstones. Without light, they are nothing but rocks.


The interplay of light is literally what defines a gemstone, and the majority of terminology related to a gemstone relates to what light does to it. But what a confusing lot of strange words are used to define the effects of light on a stone!


If you’re like us, you need a ‘for dummies’ version of all the scientific language. Here’s our attempt at it.


Basically, light striking a gemstone defines:

  • Its color

  • Its visual effects – how it appears to you when you look at it

  • Its ‘special effects’


And these characteristics are in turn influenced by a gemstone’s:

  • Cut

  • Setting


In this article, we’ll explore a little about all these effects of light on stones. Many of us, being the creative artisans we are, select gemstones for our designs based on what is pleasing to us. That works.


But if we add to that a better understanding of how light plays on a stone, we add the ability to better plan our designs to select the gemstones that most enhance the design, and conversely, to start with a stone and create a design that takes best advantage of its characteristics.



WHAT CREATES GEMSTONE COLOR?


If you took physics in school, you may remember learning about how light creates color, but let’s do a simple refresher. Daylight (white light) is actually composed of all the different colors of the spectrum.


When white light shines on a solid material, like a leaf, part of the spectrum is absorbed, while the rest of it is reflected back to the eye (the green in the case of the leaf). The same is true in gems. A ruby is red because all the other colors of the spectrum are absorbed by the stone, reflecting only the red for us to perceive.


But what causes that missing absorption of certain colors of the light spectrum? For our purposes, the main reason is the presence of ‘impurities’ in the stone. Like a leaf in which chlorophyll is the ‘impurity’ that does not absorb light, gemstones are defined by the ‘impurities’ that cause their color.


Those impurities can be other elements, like iron or chromium, or they may be imperfections in the stone’s structure. For example, when traces of chromium are in the mineral beryl, the color green is reflected and you have an emerald! But a stone like turquoise gains its color from its structural imperfections.


The same ‘impurity’ has differing effects on different minerals. Chromium, for example, creates red in corundum (ruby) but green in beryl (emerald). This is because the chromium absorbs light differently when it is in different minerals.


There are other factors in creating color, such as electron charge transfers, but this brief overview should suffice.



VISUAL EFFECTS OF A GEMSTONE: LUSTER, BRILLIANCE AND FIRE


Beyond color, we also see other visual characteristics when we look at a gemstone. We describe a gemstone as ‘shimmering’ or ‘sparkly’ or ‘lustrous’ or ‘iridescent’ or many other terms to describe what we see. These visual cues, too, are created by light playing on the stone. And they have special – and fairly specific – names in the world of gemology.


The main visual characteristics of most gemstones are luster, brilliance, and fire.



LUSTER


The primary of these is luster. It applies to all minerals, including gemstones, and describes the appearance of the stone when light is reflected off its exterior surface.

There are specific terms you’ll see to define a stone’s luster.


Types of Gemstone Luster, copyright © Lucidity Gemstones

You can’t apply a specific luster name to a gemstone family. Depending on how a stone is cut and polished, its luster may vary. As an example, a peridot, exhibiting a greasy luster in the rough, can be described as vitreous if it is well-cut and polished. And a well-polished opal may be pearly rather than waxy.



BRILLIANCE AND FIRE


Brilliance and fire are different from luster in two respects: 1) the terms only apply to transparent faceted gems, and 2) they describe how light reflects up to the eye from the interior of the stone. These two visual effects are a function of the faceting in the stone as well as the stone’s own characteristics. You’ll most often see these terms applied to diamonds, but they apply to other gemstones as well.


There are two main factors that influence how we perceive the visual effects of brilliance and fire: reflection and refraction.


Gemstone luster, brilliance and fire, copyright © Lucidity Gemstones

Reflection is the amount of light that bounces back from inside the stone. The more light that bounces back, the better the cut and the quality (inclusion-free) of the stone.


Refraction is how the stone bends light in different directions and returns a prism of colors to your eyes. It is referred to as the fire of a stone, and it is measured by a refraction index (RI). The higher the RI, the more ‘fire’ or rainbow effect you’ll see in the stone.


A higher RI stone will reflect light from a much wider range of angles than the lower RI stone, adding to the brilliance of the gemstone. A lower RI gem will appear dull more quickly when tilted. Diamond, for example, has a high RI, and many garnets have a fairly high RI, with demantoids being the highest. The International Gem Society (IGS) has a refraction index online for those interested in exploring further.



GEMSTONE SPECIAL EFFECTS CREATED BY LIGHT PLAY


Light also brings out ‘special effects’ in some stones. Gems that display unusual optical properties are known as phenomenal gemstones. Here’s where we find terminology you’ve probably seen or heard and have poorly understood.



COLOR 'SPECIAL EFFECTS'


Some stones exhibit variations in color that have special names.