Optical Properties

Introduction

Gemstones are renowned for their captivating optical properties, which contribute to their brilliance, colour, and unique visual appeal. These properties arise from the way light interacts with the gemstone's crystal structure and chemical composition. Understanding these optical properties is crucial for appreciating, identifying, and grading gemstones.

Diaphaneity

Diaphaneity describes the degree to which light can pass through a gemstone. It is classified into three categories:

  • Transparent: Gemstones that allow light to pass through freely, allowing objects on the other side to be clearly seen. Examples include diamond, ruby, and sapphire.
  • Translucent: Gemstones that allow some light to pass through, but objects on the other side become distorted or obscured. Examples include opal, chalcedony, and moonstone.
  • Opaque: Gemstones that do not allow any light to pass through, appearing solid and blocking all light. Examples include obsidian, magnetite, and hematite.

Colour

Colour is perhaps the most striking optical property of gemstones, attracting attention and influencing their perceived value. The colour of a gemstone arises from the absorption or reflection of specific wavelengths of light by its chemical composition and crystal structure. Some gemstones, like ruby and emerald, exhibit a single, intense colour, while others, like sapphire and garnet, display a range of colours.

Streak

Streak is the colour of a fine powder produced by rubbing a gemstone against a hard abrasive surface. It provides a diagnostic tool for identifying gemstones, as it is not influenced by surface features or impurities. For instance, pyrite, which has a brassy appearance, produces a greenish-black streak, while hematite, which appears reddish-brown, produces a red streak.

Lustre

Lustre describes the appearance of a gemstone's reflected light. It is a fundamental property of gemstones, influencing their brilliance and visual appeal. Lustre is classified into various categories:

  • Adamantine: Gemstones that reflect light in a brilliant, mirror-like manner, reminiscent of polished metal. Examples include diamond, corundum (ruby and sapphire), and spinel.
  • Sub-adamantine: Gemstones that exhibit a high degree of brilliance but slightly less intense than adamantine lustre. Examples include zircon, chrysoberyl, and garnet.
  • Vitreous: Gemstones that reflect light in a glassy manner, resembling the shine of glass. Examples include quartz, topaz, and beryl (emerald and aquamarine).
  • Metallic: Gemstones that reflect light in a manner similar to polished metal, often with a high degree of reflectivity and a diffuse dispersion of light. Examples include pyrite, hematite, and galena.
  • Pearly: Gemstones that exhibit a soft, iridescent sheen, resembling the shimmer of pearls. Examples include pearl, moonstone, and some varieties of opal.
  • Silky: Gemstones that exhibit a soft, fibrous appearance, resembling the texture of silk. Examples of gemstones with silky lustre include cat's eye chrysoberyl, fibrous malachite, and some varieties of nephrite jade.
  • Greasy: Gemstones that exhibit a dull, oily appearance, resembling the surface of grease. Examples include opaque turquoise and some varieties of jadeite jade.
  • Resinous: Gemstones that exhibit a soft, waxy appearance, resembling the surface of resin. Examples of gemstones with resinous lustre include amber and some varieties of opal.
  • Waxy: Gemstones that exhibit a soft, greasy appearance, resembling the surface of wax. Examples include opaque white opal and some varieties of nephrite jade.
  • Dull: Gemstones that exhibit a lack of reflectivity, resembling the appearance of an unpolished surface. Examples of dull gemstones include some varieties of agate, jasper, and obsidian.

It is important to note that the distinction between these types of lustre can be subtle and may vary depending on the specific gemstone and its characteristics.

Optical Class

Optical class refers to the behaviour of light as it travels through a gemstone. It is characterized by the number of directions in which light can travel through the crystal.

  • Isotropic: Gemstones in which light travels at the same speed in all directions. Examples include diamond, garnet, and spinel.
  • Uniaxial: Gemstones in which light travels at different speeds in two directions, one perpendicular to the optical axis and the other along the optical axis. Examples include ruby, sapphire, and quartz.
  • Biaxial: Gemstones in which light travels at different speeds in three directions, with two perpendicular to each other and the third oblique to both. Examples include tourmaline, opal, and orthoclase feldspar.

Refractive Index

Refractive index is a measure of how much a gemstone bends light as it passes through. It is a numerical value that compares the speed of light in a vacuum to its speed in the gemstone. Gemstones with high refractive indices, such as diamond (2.42) and corundum (1.77), bend light more than those with low refractive indices, such as fluorite (1.43) and quartz (1.54). High refractive indices contribute to a gemstone's brilliance and sparkle.

Birefringence

Birefringence, also known as double refraction, is the property of some gemstones to split a single beam of light into two separate beams. This occurs in gemstones with an anisotropic crystal structure, where the speed of light varies in different directions. Examples of birefringent gemstones include calcite, tourmaline, and opal.

Dispersion

Dispersion, also known as fire, is the property of a gemstone to separate and spread white light into its constituent colours (the colours of the rainbow) as it passes through the gemstone. It is caused by the different wavelengths of light traveling at different speeds through the gemstone. The higher the dispersion of a gemstone, the more it will spread the colours of the rainbow, creating a dazzling display of brilliance and sparkle.

How Dispersion Occurs

When white light enters a gemstone, it interacts with the atoms in the gemstone's crystal structure. These atoms cause the light to bend and slow down, and the amount of bending and slowing down depends on the wavelength of the light. Shorter wavelengths, such as blue, bend and slow down more than longer wavelengths, such as red.

As the light passes through the gemstone, the different wavelengths of light emerge at slightly different angles, creating a separation of colours. This separation is what we perceive as dispersion.

Dispersion and Brilliance

Dispersion is a key factor in contributing to a gemstone's brilliance. Gemstones with high dispersion, such as diamond, produce a dazzling display of rainbow-like colours, while gemstones with low dispersion, such as quartz, may appear more subdued.

The cut of a gemstone can also influence its dispersion. A well-cut gemstone with many facets will reflect and refract light more effectively, enhancing its dispersion and overall brilliance.

Some of the gemstones known for their high dispersion include:

  • Diamond
  • Zircon
  • Fluorite
  • Sphalerite
  • Moissanite

Measuring Dispersion

Dispersion is measured using a tool called a refractometer, which measures the refractive index of a gemstone for different wavelengths of light. The difference between the refractive index for red light and the refractive index for violet light is known as the gemstone's dispersion value. Higher dispersion values indicate greater dispersion.

Dispersion in Gemstone Grading

Dispersion is one of the factors considered when grading gemstones. Gemstones with high dispersion are often considered more valuable and desirable due to their captivating brilliance and sparkle.

Pleochroism

Pleochroism is the ability of some gemstones to exhibit different colours when viewed from different angles. This arises from the selective absorption of light by the gemstone's crystal structure. Gemstones with strong pleochroism, such as alexandrite and tourmaline, can change colour dramatically depending on the viewing direction.

Conclusion

The optical properties of gemstones play a crucial role in their beauty, value, and identification. Understanding these properties allows gemologists, jewellers, and gemstone enthusiasts to appreciate the unique characteristics and intricacies of these precious stones.