There’s more to gemstones and diamonds than just jewelry cases. Gemstones hold an entire world of science beyond consumerism, marketing and sales goals. It’s not easy to understand. The gemstone refractive index is part of the equation.
In this guide, we’ll explore gemstone refractive index and answer some of the most frequently asked questions including:
- What’s double refraction?
- How does gemstone refractive index affect brilliance?
- How can you measure a gemstone’s refractive index?
Gemstone Refractive Index
One of the most important optical properties of gemstones and diamonds is light. How light passes through a gemstone helps determine its fire and overall brilliance. Light tends to bend when it passes through a stone. This process is usually known as refraction.
What Is Gemstone Refractive Index?
Light bends when it passes through a stone and also slows down. The gemstone refractive index also known as the RI, is typically the ratio of the speed of light between the air and the stone. Liquids have refractive indexes as well.
When it comes to liquids, the refractive index is ration between the speed (velocity) of the air and the liquid.
What’s A Higher Refractive Index?
Gemstones with higher refractive indexes tend to display a stronger light performance. Some people find gemstone’s fire with higher refractive index a bit overwhelming while others think it’s absolutely stunning.
It’s a lot easier to cut brilliant gemstones from a gem material with a higher refractive index. Quartz can appear dull especially the gem cutter doesn’t facet it correctly owing to its low refractive index.
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What’s Gemstone Windowing?
Gemstones that have lower refractive indices are usually more susceptible to gem cutting obstacles which is known as windowing. Windowing can happen to all gemstones as long as they’re not cut correctly. It’s almost unavoidable with low refractive indices gemstones.
In most cases, the gemstone cutter will not only want to cut a significant sized stone but also make it look beautiful as possible. However, the carat weight can outweigh the cut especially when it comes to colored gemstones. It’s a different story altogether for diamonds.
The color scale of colored gemstones is quite different from that of diamonds and their lab created counterparts. For this reason, you might have to eye colored stones physically or through the famous 360-degrees viewer.
Windowing tends to occur when the bottom of the gemstone is shallow and its table larger. In this case, the center stone is bound to look lifeless and dull. The gemstone refractive index shows the angle that the light passes through the stone in question. The critical angle will guide the gem cutter on how to facet the stone.
The light will pass through the bottom of the stone and not return to the eye if the gem cutter undershoots the critical angle. You can place the gemstone on a piece of paper to ascertain whether or not you have a loose windowed stone.
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The gemstone is probably windowed if you can see through it. A gemstone that’s cut for brilliance usually breaks up light without showing through a piece of paper.
Gemstone Refractive Indices Values
The speed at which light passes through gemstones vary from one stone to another. This means that different gemstones have different refractive index values. What’s more, there’s a wide variety of refractive indices since there are lots of gemstones out there.
Here’s a highlight of different refractive indices of some of the most common gemstones on the market:
- Moissanite – 2.65 to 2.69
- Diamonds – 2.417 to 2.419
- Cubic zirconia – 2.15 to 2.18
- Zircon – 1.810 to 2.024
- Spessartite Garnet – 1.79 to 1.82
- Almandine Garnet – 1.77 to 1.82
- Hessonite Garnet – 1.730 to 1.757
- Pyrope Garnet – 1.720 to 1.756
- Spinel – 1.712 to 1.762
- Tourmaline – 1.614 to 1.666
- Topaz – 1.609 to 1.643
- Emerald – 1.565 to 1.602
- Quartz – 1.544 to 1.553
Most gemstones albeit with different color varieties usually have the similar refractive index values like other in the same family. Morganite, emerald and aquamarine have the same refractive indices. The same goes for amethyst gems, white quartz, smoky quartz and rose quartz. However, the rule doesn’t apply to garnet gemstone species.
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Garnets are darkish red and the birthstone for those born in January. Unlike other gemstones, garnets come in different species as opposed to varieties. Spessartite garnet, hessonite garnet and pyrope garnet all have different refractive index values.
Importance of Gemstone Refractive Index
Gemstone refractive indices are important for many different reasons. For starters, the refractive index value help ensure that the stone is faceted correctly.
Here area few reasons why gemstone’s refractive index is important. Let’s get straight into it, shall we?
- Refractive index can affect the light performance of a gemstone
Light tends to leak out of the stone if the gem cutter doesn’t facet it correctly as per the critical angle of the gemstone’s refractive index. Plus, it won’t return back to the eye. The stone will appear lifeless and dull if light leaks out. In some cases, the gemstone looks blurry.
- The refractive index value helps identify gemstones
Refractive index values are equally important when it comes to identifying gemstones. The value is quite handy especially when you can’t tell the difference between two almost identical gemstones. What’s more, you can tell a real diamond from an imitation using refractive indices.
- Refractive index affects the clarity of a gemstone
The crystal structure usually comes in contact with impurities and other crystals when gemstones form underneath the earth. Some impurities cause natural inclusions while others change the color of the crystal structure.
Every gemstone has inclusions, natural or otherwise. However, some inclusions aren’t quite noticeable and can only be seen under magnification. The price and value of diamonds increase depending on their clarity grade.
Some colored gemstones contain other gemstone crystals in them. If the crystal inclusions have the same refractive index values, then the inclusions might not be visible to naked eyes. For instance, if a small piece of aquamarine is trapped in an emerald gemstone, it will be less noticeable. This is because the refractive index of aquamarine and emerald is the same since they’re beryl mineral varieties.
However, pyrite crystal usually found in Colombian emeralds appears darker owing to the difference in value of the refractive index.
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What’s Double Refraction?
Refraction is when light passes through a stone from one medium to another then bends back out. However, when a gemstone allows light to pass through it and bend back out effortlessly in one color, it’s known as isotropic gemstone.
Amorphous gems and cubic crystals like amber and opals are isotropic gemstones. They’re known as singly refractive stones.
When it comes to double refraction, two or three beams of light reflect out from the stone in different directions. In this case, each light usually has its own refractive index once they scatter, and that’s what is referred to as double refraction.
The difference between the double refraction index values is known as birefringence.
Double Refraction Index Values/ Birefringence
Here’s a highlight of some of the most common gemstones on the market along with their double refractive indices (birefringence). However, some gemstones don’t have double refractive index values.
- Diamonds – Isometric cubic crystals have a relatively weak birefringence.
- Ruby/Sapphire – 0.008
- Spessartite Garnet – None
- Hessonite Garnet – None
- Pyrope Garnet – None
- Almandine Garnet – None
- Cubic Zirconia – None
- Spinel – None
- Zircon – 0.002 to 0.059
- Tourmaline – 0.014 to 0.032
- Topaz – 0.008 to 0.016
- Emerald – 0.006
- Quartz – 0.009
- Moissanite -0.313
Measuring Gemstone Refractive Index
Fortunately, you don’t have to worry about how to measure the refractive index of a gemstone if you’re not into the business of identifying gemstones.
How Scientists Measure Gemstone Refractive Index
Scientists and gemologists use a refractometer to measure gemstone refractive index. It’s one of the most important tools that you need in a gem cutter’s or jeweler’s toolbox.
The refractometer helps measure the refractive index values of the wavelength of light passing through the gemstone. However, different wavelengths have different refractive indices. The refractometer resembles a microscope and provides the refractive index reading of the stone by showing it on the eyepiece.
A gemstone refractometer is one of the most important gemology tools for identifying gemstones. However, it doesn’t let you know anything about the stone. This means that you’ll not be able to tell whether the stone in question is natural or lab created since it only shows you birefringence and refractive index values.
However, if you’re looking to identify the origin of the stone, then a legit grading report will come in handy. Most refractometers don’t usually measure the refractive index of diamonds moissanite stones and cubic zirconia.
Gemstones with higher refractive indices are high for the readers. Besides, refractometers don’t go beyond 1.81.
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Final Thoughts – Gemstone Refractive Index
It’s okay if this information about gemstone refractive indices is a lot for you. You might understand the basics even though you may not be conversant with specific gravity, different light wavelengths and absorption spectrum.
The gemstone refractive index is the ratio of the speed of light between the air and the stone. This means that light bends back out when it passes through a stone. What’s more, every beam has its own refractive index value.
The gemstone refractive index value impacts different aspects of the stone. Plus, it helps gem cutters facet the stone correctly along the critical angle where the light passes through it. It also affects the appearance of gem crystal inclusions in the stone.
A gemstone refractometer is one of the most important gemology tools especially if you want to know the refractive index values of different gemstones. However, refractometers don’t go beyond 1.81 and neither does it help identify natural gemstones from their lab created counterparts.
