Double Refraction or "Doubling"

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Among gemstones' amazing phenomena is double refraction, or “doubling,” and is one of the most intriguing and visually stunning. If you've ever gazed into a gemstone and noticed two of everything, you’ve witnessed the magic of double refraction.

In this blog post, we’ll delve into the science behind double refraction, explain why it happens, and explore how gemologists and enthusiasts use this phenomenon to identify gemstones.

So what is Double Refraction?

Double refraction, also known as birefringence, is an optical phenomenon that occurs when light enters certain types of gemstones and splits into two separate rays, traveling at different speeds and directions. This results in a doubling effect, where images or text viewed through the gemstone appear duplicated. The degree of doubling depends on the gemstone’s birefringence value—a measure of how much light is split.

  • ZR0106 blue cambodian zircon cushion

    This Zircon isn't out of focus! The Doubling effect is prominantly displayed in this macro photograph.

    5.44ct Vivid Blue Zircon 

  • Why does Double Refraction occure?

    To understand double refraction, we need to dive into the structure of gemstones. Most gemstones are crystalline solids, meaning their atoms are arranged in a highly ordered pattern. However, not all crystals interact with light in the same way.

    • Isotropic Crystals: Gemstones like diamonds, garnets, and spinels have a uniform internal structure that interacts with light equally in all directions. These are called isotropic stones, and they exhibit no double refraction.
    • Anisotropic Crystals: In contrast, most gemstones, including rubies, sapphires, zircons, and moissanite, belong to the anisotropic category. Their internal structures cause light to split into two rays, each refracted differently depending on the gemstone's optical axes.

    This splitting occurs because anisotropic crystals have varying refractive indices (a measure of how much light bends) in different directions.

Identifying Gemstones Using Double Refraction

Gemologists use double refraction as an essential tool in identifying and distinguishing gemstones. Here's how it plays a role:

  1. Polarizing Microscope: Gemologists use tools like polariscopes to observe how light interacts with a gemstone. Anisotropic gemstones will exhibit double refraction under crossed polars, revealing diagnostic optical properties.
  2. Magnified Observation: Doubling can sometimes be seen with a loupe or macro photography, especially in gemstones with strong birefringence, like zircon, peridot, and moissanite. Observing the back facets of the stone for doubling is a common practice.
  3. Refractometer: The refractometer measures the refractive indices of gemstones, including birefringence, to confirm their identity.

Famous Gemstones with High Birefringence

0.043 (strong doubling)

Moissanite

A lab created gemstone intended to be an alternative option to diamond with its incredible brilliance and fire. Moissanite has strong doubling which can help differentiate it from diamond.

ZR0134 champagne colored zircon trillion

0.059 (strong doubling)

Zircon

Often confused with diamonds due to its brilliance, zircon's strong to very strong doubling makes it easy to distinguish from its counterparts.

0.172 (using birefringence blink method)

Calcite

Exhibits dramatic doubling and is often used in optical experiments to demonstrate birefringence.

A low Mohs Scale hardness of 3 makes it unsuitable for most jewelry.

PD0109 green pear peridot

0.014 - 0.016 (weak doubling)

Peridot

Known for its vibrant green color, peridot displays doubling in its facets when viewed closely.

oval peach tourmaline

0.08 - 0.010 & 0.018 - 0.020 respectively (weak doubling)

Topaz & Tourmaline

These gems also exhibit double refraction, although to a lesser degree than zircon or calcite.

Appreciating the Beauty of Doubling

While doubling can serve as an identifier for certain gems, it also adds a unique dimension to their appeal. Observing double refraction is like peering into a gemstone’s secret world—a reminder that the natural forces shaping these gems are as extraordinary as the gems themselves. It can also add additional life and scintillation to the gemstone as light glitters among the many additional reflections within the gem.

Double refraction is more than just a scientific curiosity; it’s a phenomenon that enhances the allure and mystery of gemstones. Whether you're a gem enthusiast or someone exploring the field of gemology, understanding and appreciating the beauty of doubling brings you closer to the heart of these treasures from the earth.

So next time you’re admiring a gemstone, take a closer look—you just might find a doubled world hiding within it!