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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Jet

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Jet is a black organic substance which is a fossilized variety of brown coal, formed as a result of the compaction of driftwood over millions of years.

Jet has a dull lustre in rough form while vitreous lustre after polished. It is a stone with low S.G. (1.20 – 1.30) but high R.I. (1.66). It is the key identifying feature of jet for separating it from its simulants.

Chemical Composition : Carbon with impurities but not a mineral.

Classification / Type : 

A compact form of coal (further heat and pressure makes it more tough)

Crystal System / Forms : 

None. Amorphous.

Cuts & Uses : 

Carvings, beads, cabochons, etc.

Dispersion : None.
Hardness : 2.5 - 4
Lustre : Dull in rough; vitreous when polished.
Magnification : 

None.

Optic Character : 

Isotropic (S.R.)

Pleochroism : None.
Refractive Index / Birefringence : 

1.66 (approx.) / Nil.

Simulants (with separation tests) :
  • Anthracite (black streak)
  • Glass (R.I., S.G., inclusions)
  • Plastic (R.I., hot point, structure)
  • Black stained chalcedony (R.I., S.G., banding)
  • Obsidian (R.I., S.G., inclusions)
  • Vulcanite – hard rubber (rotten egg smell to a hot point)
Sources : 

England, Spain, France, U.S.A., Germany, Russia.

Specific Tests :
  • Gives a strong coal or oily odor to a hot point.
  • Brown streak color on a porcelain plate.
Spectrum : 

None.

Synthesis : 

None.



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