Gemstone Windows and Dark Patches: What to Look for in a Cut Gem

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AI-generated educational illustration. Conceptual only; not a photograph of an actual specimen or location. A coloured gem may have a pale, see-through centre or areas that look almost black. Both affect its appearance, but they do not have the same cause. What is a window? A window is an area where light passes through the gem instead of being returned towards the viewer. You may see the background through it, especially near the centre. Cutting angles and the material's refractive index—how it bends light—help determine whether this happens. There is no single ideal depth for every gemstone. Why dark patches need a closer look Darkness can have several causes. The material may absorb a lot of light. Some facets may reflect dark surroundings or the observer. Extinction is another cause, involving light that is not returned to the viewer after internal reflection. For that reason, calling every black patch “extinction” can be misleading. A simple viewing routine Look at the gem fac...
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introduction to gemology

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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

 

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