Diamond Rocks UK Logo

The lowdown on the optical properties of gemstones

Gemstones have fascinated humankind since the beginning of our existence with their colouring, luminescence and overall brilliance. Whether you’ve had an interest in gemstones for years or you’re just getting into gemology, there’s a lot to learn.

One of the areas that many people have questions about is the optical properties of these stones. By and large, optical properties are the most important aspect of a gemstone, because they allow gemologists and others to classify stones based on their appearance. Keep reading to learn more about these unique properties that make gemstones one of Mother Nature’s greatest gifts.


What are optical properties?

Simply put, the term ‘optical properties’ refers to what the stone looks like. Specifically, these properties are defined by the way light interacts with a gemstone and how differing aspects are assessed. Some of these properties, such as colour, lustre and transparency, are apparent simply by looking at a stone. Other optical properties, however, require special tools in order to be measured. These include dispersion, refractive index, and several others that are studied in laboratory gemology and used to classify and rate particular gemstones.



Perhaps the most recognised optical property of any gemstone is its colour. Colour can be used to quickly, visually classify gemstones. While most people associate red with rubies, green with emeralds, blue with sapphires and so on, most stones actually contain a wide range of colours. Some of these may be visible to the naked eye, but others are not and can only be seen with the use of a spectroscope.

A spectroscope measures what colours of light a gemstone absorbs. In fact, this is how gems get their colouring – through light absorption. The process is known as selective absorption and works like this: white light, which contains the full colour spectrum of the rainbow, enters a gem. Some of this light is absorbed, but what is not determines the colour of the stone. While pigment within the gem itself may play a small role, most gem colouring is largely determined by this reaction to white light.  

A note on colour: When identifying gems, colour cannot be used as a standalone measurement. If it were, garnets and rubies would be confused an awful lot, along with other stones that exhibit the same colouring. A spectroscope, the tool mentioned above, is used to identify all the colours within a gem – even the ones that cannot be seen by the naked eye. A spectroscope allows the viewer to see each wavelength of colour in the stone, giving a true measurement of a gem’s colouring and enabling gemologists to determine rubies from garnets.



Lustre refers to the brilliance of a gem and how it reflects light. While there is no scientific way to determine a stone’s lustre, there are several types of lustres that are widely recognised, including metallic, dull, waxy, vitreous, resinous and more. Gems are not usually identified using lustre, as the lustre of a particular stone can appear different to different people. In fact, vitreous gems are indistinguishable from one another in that respect.

However, lustre is still an optical property that many people are interested in, and the best way to determine it is to view the gem in a well-lit space, taking special note of any uncut portions of the stone.



Aside from a gem’s colour, its transparency is its most easily recognisable optical property. Transparency is defined as the amount of light that can pass through a stone. There are three degrees: transparent, translucent and opaque. Transparent stones allow light to pass through, while translucent gems only let some light pass and opaque stones don’t allow any light to pass through. Transparency will vary from stone to stone, even those of the same variety. For example, on emerald may exhibit almost complete transparency, while another is completely opaque. Flaws and impurities in a stone will reduce its transparency. No special instruments are required to determine transparency. Simply hold the gem in a strong light and visually assess how much light is able to pass through the stone.



Dispersion is directly linked to fire, which is a term associated with gemstones (and diamonds in particular) with which most people are familiar. Dispersion occurs when white light enters a gem and splits into the various hues of the colour spectrum. This happens with transparent, colourless gems that have been cut and faceted. The light reflects off the stone’s inner structures, resulting in the fire, or sparkle, that so many covet. Diamonds have the greatest fire of all natural gems.


Refractive index

Refractive index, or RI as it’s referred to in gemological circles, is all about the speed of light. You probably already know that light speed varies depending on the environment, object, or substance. When light moves from one dimension, such as air, to another, such as water, it refracts when entering the second dimension. This refraction, or bending of the light, is due to differing speeds of light in the different dimensions. With gems, light is refracted in transparent stones, since the speed of light in the stone is different than that in the air. The refractive index of a particular gemstone determines its brilliance.

The vast nature of the optical properties of gemstones is fascinating. While there are a few others that we didn’t cover, the properties listed above are the ones that you will most likely encounter in your study of gems. Colour, lustre and transparency, in particular, are the most commonly discussed properties, as these are the ones that can be determined without the use of specialized instruments.

However, the other properties are just as important, especially when it comes to accurately classifying or rating stones. Whether you’re a budding gemologist or just an average person with an above average interest in gems, the descriptions above will help you to better understand the optical properties of gemstones – and what makes them the treasures they are.

This website is using cookies. More info. That's Fine