What is Double refraction?

Understanding Double Refraction: An Insight into Optical Phenomena

Introduction

Double refraction was discovered by Bartolin, who observed that when a ray of unpolarized light is incident on a calcite crystal, two refracted rays are produced. This phenomenon, known as double refraction, results in two images of a single object. This optical phenomenon is exhibited by several other crystals like quartz and mica.

The Phenomenon of Double Refraction

When an ink dot on a sheet of paper is viewed through a calcite crystal, two images will be seen. Rotating the crystal causes one image to remain stationary while the other rotates around it. The stationary image is known as the ordinary image (o), produced by the refracted rays which obey the laws of refraction. These rays are known as ordinary rays. The other image is the extraordinary image (e), produced by the refracted rays which do not obey the laws of refraction. These rays are known as extraordinary rays.

Behavior of Rays Inside the Crystal

Inside a double refracting crystal, the ordinary ray travels with the same velocity in all directions, while the extraordinary ray travels with different velocities along different directions. A point source inside the crystal produces a spherical wavefront corresponding to the ordinary ray and an elliptical wavefront corresponding to the extraordinary ray. There is a particular direction inside the crystal in which both rays travel with the same velocity, known as the optic axis. The refractive index is the same for both rays, and there is no double refraction along this direction.

For more detailed insights on double refraction and related optical phenomena, you can visit the following resources:

• Physics Hypertextbook on Double Refraction

• University of Oregon's Optical Phenomena

The Polariscope and its uses

The Polariscope 

Polariscope is a quick and useful tool that will help us to identify the optical nature of a gemstone and conclude whether it is singly or doubly refractive. These gems are known as isotropic or anisotropic respectively. This tool will also identify gems with a polycrystalline nature and on a few occasions, the polariscope can fully identify the gem.

 

A polariscope can be used on rough, cut, set, and loose gemstones. The only limitation is that the gemstone must have some transparency and that the setting does not inhibit us from viewing any light traveling through the stone.

There are two types of polariscope

·       A portable polariscope, which requires an additional light source

·       A bench (Table) polariscope with a built-in light source, which plugs straight into the mains.

A polariscope is made up of two polarizing filters.

 In the portable polariscope, these filters are fixed at a 90-degree angle to one another. To use the portable polariscope, open the polariscope fully by moving the two filters back on themselves to lock it into place. Put the polariscope on your light source and turn the light source on. The test results will be most easily seen in an area without surrounding light. When looking through the polariscope, the bottom stage will appear dark. This indicates the polarizing filters are at 90 degrees to one another, which is also known as the crossed position.

 In a bench polariscope, the top filter is rotatable and must be turned to be fixed at 90 degrees. With a bench polariscope, turn the top filter until the bottom stage appears its darkest.

 

When a gemstone is placed in between these two filters and turned, it will create one of four basic patterns of light and dark depending on the optical nature of the gem. This piece of equipment does require practice at first to ensure that you can recognize the patterns effectively.

1.       If the gemstone appears dark on one full 360-degree rotation, the gemstone is singly refractive and will either be cubic or amorphous. Gems that show this pattern include garnets, spinel, diamond, and glass.

2.       If a gemstone is a mix of light and dark throughout the entire 360-degree rotation, this is another test result for singly refractive gems, so either cubic or amorphous, but these gems also contain strain within its structure. This is known as anomalous extinction effects (AEE) or anomalous double refraction (ADR).

3.       If the gemstone appears light then dark four times in one full 360-degree rotation, the gemstone is doubly refractive and will be in one of the six doubly refractive crystal systems. Please note that the gem may not go dark all at once. The darkness may pan across the stone as it is rotated, but it will go completely light at four points during this rotation. Gems that show this pattern include corundum, topaz, zircon.

Noted:  iridescence is seen when looking through the polariscope, this indicates that you are looking along the optic axis of a doubly refractive gem. To be sure that this is the case and that you're not viewing iridescence in a fracture or other inclusion, turn the gem on a different axis and test again to see the light and dark patterns.

4.       If a gemstone appears light on one full 360-degree rotation, the gemstone is polycrystalline. Gems that show this pattern include polycrystalline quartz such as agate, aventurine quartz, and chalcedony. and beryl.

Gemology Quiz

Gemstone Quiz

1. 1. Which of the following is a defining characteristic of organic gems?

   A) Created by geological processes
   B) Formed through natural weathering
   C) Made by living organisms
   D) Originates from volcanic activity

2. 2. Which gemstone is considered organic?

   A) Sapphire
   B) Coral
   C) Diamond
   D) Topaz

3. 3. What is the primary factor that qualifies a material to be considered a gemstone?

   A) Weight and size
   B) Beauty
   C) Age and origin
   D) Transparency and brilliance

4. 4. Which of the following is a measure of a gemstone's ability to resist scratching?

   A) Luster
   B) Toughness
   C) Hardness
   D) Brilliance

5. 5. Which scale is commonly used to measure the hardness of gemstones?

   A) Richter Scale
   B) Kelvin Scale
   C) Mohs Scale
   D) Celsius Scale

introduction to gemology

Gemology: The Study of Gemstones

Introduction

Gemology is the study of gemstones, including their properties, locations, and origins. It is often studied by individuals in the jewelry business, including business owners, buyers, designers, and appraisers. Antique dealers also study gemology to identify and describe the properties of gems and gemstones.

What is a Gemologist?

A gemologist is someone who can identify gemstones using specialized instruments.

What is a Mineral?

A mineral is defined as a substance that:

• Is naturally occurring
• Is inorganic
• Has a defined chemical composition
• Possesses an orderly atomic structure

What is a Gemstone?

A gemstone is a natural, inorganic or organic substance that has substantial beauty, rarity, and durability.

Types of Gems

1. Organic Gems: Made by living organisms, examples include pearls, coral, jet, ivory, shell, and amber.
2. Inorganic Gems: Created by geological processes and usually dug from the ground.

Gemstone Characteristics

1. Beauty: To be considered a gem, a material must possess beauty, which can depend on color, transparency, luster, brilliance, pattern, optical phenomena, and distinctive inclusions.
2. Rarity: The rarity of a gemstone adds to its value. For example, taaffeite is highly valued because it is rare, despite its pale reddish-violet color.
3. Durability: This depends on three factors:
• Hardness: The ability to resist scratching, measured on the Mohs scale from 1 (talc) to 10 (diamond).
• Toughness
• Stability