Gemstone Refractive Index: What It is and Why It’s Important

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There's a lot more to diamonds and gemstones that goes beyond the jewelry case. Behind marketing, sales goals, and consumerism, gemstones hold a world of science. It can be difficult to understand.

The Gemstone refractive index can be one of those things. In this Learning Guide, we'll go over gemstone refraction plus answer these questions: 

Gemstone Refractive Index
  • What is double refraction?
  • How does one measure a gem’s refractive index?
  • How the refractive index affects brilliance?

Light is a very important optical property of a diamond or any gemstone. The way light passes through a stone determines its overall brilliance and fire. When light passes through a stone, the light bends. It is known as refraction.

What is a Gemstone's Refractive Index?

Not only does light bend when it passes from the air into the stone, it slows down too. The gemstone's refractive index (abbreviated RI) is the ratio of the speed of light between the air and stone.

Other things have refractive indexes too, like liquids. In that case, the refractive index would be the velocity (speed) of the light between the air and liquid.

What Does A Higher Refractive Index Mean?

If a gemstone has a higher refractive index, it means that it displays a strong light performance. The fire of a gemstone with high refractive index may appear overwhelming to some, yet absolutely stunning to others. 

It's easier to cut brilliant gems out of gem material with a high refractive index. Quartz has one of the lowest refractive indices, which means it can appear very dull if not faceted correctly by a gem cutter.

What is Gemstone Windowing?

Stones with a lower refractive index like quartz is susceptible to a gem cutting obstacle called windowing. Windowing can happen in any gemstone if it hasn't been cut correctly. However, it can be hard to avoid with gems with low refractive indices.

The goals for a gem cutter is to cut both a big AND beautiful stone. In many cases, the carat weight of a gem outweighs the cut for colored gemstones. With diamonds it's a different story.

But since colored gemstones don't have the same color scale as diamonds and lab grown diamonds, you really have to eye colored gemstones either in person or through a 360˚ viewer.

When the table of the stone is large and the bottom is shallow, windowing occurs. The entire center of the gemstone can look dull and lifeless. The refractive index reveals the critical angle in which light passes through the stone. The angle serves as a guide for the faceting. If the gem cutter undershoots the critical angle, the light passes straight through the bottom of the gemstone without returning back to the eye.

An easy way to see if you have a loose windowed gemstone is by placing it on a piece of newspaper. If you can see through the stone, it is windowed. A gem cut for brilliance will break up light and not show the newspaper through. 

Gemstone Refractive Index Values

The light passes through different gemstones at different speeds. They each have their own refractive index value. There are many different gemstone refractive indices because there are numerous gemstones. Here are the refractive indices of some of the most well known gems:

  • Diamond: 2.417-2.419
  • Sapphire/Ruby: 1.762-1.778
  • Spessartite Garnet: 1.790-1.820
  • Almandine Garnet: 1.770-1.820
  • Pyrope Garnet: 1.720-1.756
  • Hessonite Garnet: 1.730-1.757
  • Emerald: 1.565-1.602
  • Topaz: 1.609-1.643
  • Tourmaline: 1.614-1.666
  • Cubic Zirconia: 2.150-2.180
  • Moissanite: 2.65 to 2.69
  • Spinel: 1.712-1.762
  • Quartz: 1.544-1.553
  • Zircon: 1.810-2.024

Most gemstones with different color varieties will have the same refractive index value as others in the family. Emerald, aquamarine, and morganite all have the same refractive index. The same is true for rose quartz, smoky quartz, white quartz, and amethyst gems.

morganite

Morganite

Emerald

The garnet species is the exception to this rule. Most people know garnets as the dark reddish January birthstone. But garnet doesn't have varieties, but different gem species. Hessonite garnet has a different range of refractive indices than pyrope garnet or spessartite garnet.

Why is the Refractive Index of a Gemstone Important?

There's a few different reasons why the refractive index value of a gem is important. For one, the refractive index helps diamond and gem cutters facet the stone properly.

1. Refractive Index Affects Gemstone Light Performance

If the gem cutter doesn't facet a stone according the angle of the refractive index, the light leaks out the stone instead of reflected back to the eye. If the light leaks, it causes your stone to appear dull. In some stones, it will cause them to look blurry.

2. Refractive Indices Can Help Us Identify Gemstones

Another reason why the refractive index of a gem material is also important because it helps us identify gemstones. This is helpful when a stone's identity is not detectable from other stones. The refractive index can tell you a real diamond from a fake one.

3. How the Refractive Index of a Stone Affects Clarity

When gemstones form beneath the earth, the crystal structure comes into contact with other crystals and impurities. Some impurities can cause color changes in the crystal structure and others cause natural inclusions.

Inclusions are a natural part of every gem. But not every inclusion is visible to the naked eye. Some can only be seen under magnification. In diamonds, the price and value increase based on its clarity grade.

Some colored stones have bits of another gemstone crystal trapped inside them. If these crystal inclusions have the same refractive index, those inclusions will be less noticeable to the naked eye.

For example, if there's a tiny aquamarine trapped inside of an emerald gemstone, it would be less noticeable. Both emerald and aquamarine have the same refractive index because they are both varieties of the mineral beryl. But a pyrite crystal (found in Colombian emeralds) will appear darker because the difference between the two refractive indices.

What is Double Refraction?

You already know that refraction is when light passes from one medium into another and bends back out. When the gem allows the light to pass through and bend with no issue and in one color, they're called isotropic gemstones. Cubic crystals and amorphous gems like opals and amber are also part of this. They are called singly refractive gems.

If a gemstone is doubly refractive, two beams or three beams of light reflected out in different directions. When the light is scattered like that, each one has its own refractive index. It is called double refraction. Birefringence is the difference between the the double refraction indices.

Birefringence/Double Refraction Values

Here are some of the same gemstones with their birefringence values as well. Some gemstones will have no birefringence.

  • Diamond: Isometric cubic crystal, but has been known to have very weak birefringence
  • Sapphire/Ruby: 0.008
  • Spessartite Garnet: None
  • Almandine Garnet: None
  • Pyrope Garnet: None
  • Hessonite Garnet: None
  • Emerald: 0.006
  • Topaz: 0.008-0.016
  • Tourmaline: 0.014-0.032
  • Cubic Zirconia: none
  • Moissanite: 0.313
  • Spinel: None
  • Quartz: 0.009
  • Zircon: 0.002-0.059

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How to Measure Refractive Index

If you're like me, you hate math. And if you're not like me--I'm just telling you, I hate math. I'm not trying to identify gemstones, so thankfully I don't have to worry about measuring the refractive index of a gem.

How Scientists Do It

Scientists don't do it by hand either. Gemologists use a tool called a refractometer. It is a key tool in a jewelers or gem cutter's toolbox. It can measure the refractive indices of every wavelength of light that enters the stone. Remember, every wavelength has its own refractive index.

A gemstone refractometer looks like a weird microscope. It provides a reading of the refractive index of that stone by displaying it on the eyepiece. It's arguably one of the most important gemology tools for identifying gems.

credited 丰泽一号

However, a refractometer can't tell you everything about a stone. It may show you the refractive index and birefringence values, but it won't tell you if your stone is lab-created or natural. You should rely on a legitimate grading report or gemologist to identify origin. 

Majority of refractometers won't measure diamonds, cubic zirconia, or moissanite. Gems with high refractive indexes are too high for most of the readers. Refractometers don't usually go past 1.81. 

Conclusion

It's a lot of information, I know. Gemstone refractive indices can give me a headache too. You might not understand everything above wavelengths of light, absorption spectrum and specific gravity, but I'm sure you understand the basics. 

The refractive index of a gemstone is the ratio of the speed of light between the air and the stone. When the light enters the stone, it bends back out. Each beam of light in a stone has its own refractive index.

A gemstone's refractive index can impact different aspects of a gem. It show gem cutters how to cut alongside the critical angle of light passing through a gemstone. It can also affect the appearance of other gem crystal inclusions within the stone. 

Different gem materials have different refractive indices, which make it must-have gemology tool for gem identification. Though they won't be able to tell you the ratings over 1.81 or identify natural from lab grown gems, you can learn a lot from using the refractometer.

Fortunately, you don't need the refractive index of a gemstone if you're not making a career out of gem identification. There are also other ways to finding out a stone's true identity that you don't have to do yourself. But it always helpful to know a little bit on your own! 


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