What is Gemstone Birefringence?

What is Gemstone BirefringenceWe know gemstones are sparkly and bright, but do we know exactly why? What gives gems their shiny glint, and is the process different for each gem? Well, considering light refracts differently depending on the structure of the stone, we have to examine a scientific process called birefringence to answer these questions.

We can easily spot the unmistakable sparkle of a diamond, but there are a variety of other stones with different structures that emit light differently. So, what sets them apart?

Birefringence relates to the directions light is split into when it enters a gemstone, and whether or not light is refracted once, or multiple times.

Read on as we break down the definition of gemstone birefringence, which gemstones demonstrate birefringence, the features of birefringence and how to calculate a gemstone’s birefringence. But first, we can’t address any of these details without first explaining what birefringence is, can we?

What is Gemstone Birefringence?

To best answer this question, we’ll define the terms associated with the process of birefringence:

  • Refraction: When light passes through a transparent substance or object to another, such as from air to a gemstone, it bends, or changes direction.
  • Double Refraction: Light enters into an oblique substance like a gemstone and splits into two (sometimes three) different directions.
  • Birefringence: Now we can define birefringence, which is the measurement of a gemstone’s double refraction. When light refracts into two directions, or double refracts, we use birefringence to not only measure, but also identify gemstones.

How and Why Do Gemologists Measure Refractive Index?

When light enters into a gemstone, it slows down, bends and refracts. This is a quality that gemologists need to factor into their calculations when working out a gemstone’s refractive index.
What is Gemstone Birefringence

How do Gemologists Measure Refractive index (RI)?

The process involves measuring lightspeed inside of a vacuum and dividing that measurement by the speed of light passing through a gemstone. Why measure lightspeed through a vacuum? Consider that the speed of light within a vacuum will always be faster than through a gemstone, so the RI of a gemstone will always be 1 or above. Using a refractometer, the technician can accurately measure the gem’s RI.

Why does Refractive Index matter?

RI is important because ultimately, it tells the gemologist what kind of gem they’re working with. There are hundreds of gemstones in the world, and measuring RI is an effective technique to accurately and efficiently identify gemstones.

How Does Refractive Index Relate to Birefringence?

Because there are hundreds of gemstones in the world, knowing a gem’s RI identifies the stone, as well as narrows down a class of stones which demonstrate birefringence. That’s because certain stones only have one RI and don’t double refract. If a stone doesn’t double refract, than it doesn’t have birefringence.

Refractive index helps gemologists categorize gemstones by refraction. Now that we know that a stone must double refract to showcase birefringence, which stones are we talking about?  

Which Gemstones have Birefringence?

An easy way to simplify which gemstones show birefringence is to look at the gem’s structure. If the gemstone has a crystal structure, which is considered isometric and shaped like a cube, then the axes of the structure are all the same length and as such, stones with this structure only contain one RI. A prime example of an isometric stone with one RI is the diamond. Remember that when a stone only has one RI, it doesn’t have double refraction and is thus, without birefringence.
What is Gemstone Birefringence

That means that gemstones that have any other crystal structure are going to have double refraction. These gemstones will have 2-3 refractive indices which is defined by the direction in which light enters and passes through them.

Examples of gemstones that show birefringence include Zoisite, Calcite, Emerald, Tanzanite, Ruby, Sapphire and Topaz.

What are the Visual Effects of Birefringence?

Gemstones that are birefringent exhibit the following visual effects:

  • Pleochroism - This is a color effect that means 2-3 colors are displayed. Depending on how you hold the stone, and at what angle, the pleochroism will vary.
  • Double Vision - A fascinating effect of birefringence occurs on some gemstones which emit a double vision feature. When the gem is faceted, or cut, it appears to reflect two images.
  • Fuzziness - Birefringence can also naturally cause a sort of fuzziness to appear on a gemstone, making it look out of focus.

How to Calculate Birefringence

Each gemstone has both a low and high refractive index. The gap between these two metrics is where you’ll find the gem’s birefringence. Gemstones which have a high birefringence will also have more obvious visual effects.

However, there are certain gemstones that actually have a range of values that represent their refractive index, and it relates to the axes in their structure. If a gemstone has multiple RI values, then you can calculate the range of birefringence by examining the difference between the smallest values and highest values of the gem. Rather than having a birefringence of .008, like a sapphire, it’ll have a birefringence range of 0.108-0.110, like azurite.

While there are gems that don’t double refract, like diamonds, there are many gemstones that do double refract, and even a large portion of gemstones that have a range of double refraction.

A Quick Summary on Birefringence

That wraps up our overview of gemstone birefringence. Did we answer all of your questions? While it might seem like a lot of information, there are really only a few key criteria to birefringence. Below are bullet points to summarize gemstone birefringence:

  • Birefringence occurs when light double refracts when it enters a gemstone, because the light splits into 2-3 different directions.

  • A stone must have more than one refractive index to show birefringence.

  • Visual effects of birefringence include pleochroism, which relates to multiple colors showing at different angles of the gemstone, fuzziness, and double vision.

  • Some of the most popular double refractive gemstones which show birefringence are rubies and emeralds.

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