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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Gemstone Hardness vs Toughness: Why a Hard Gem Can Still Break

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A high Mohs number does not answer every question about a gemstone. For everyday jewelry, you also need to think about breakage and the stone's response to heat and chemicals.

Moonstone showing a pale sheen
Moonstone is a feldspar gem that needs gentle care. Photo: Simon Eugster, CC BY-SA 3.0, via Wikimedia Commons. Image unchanged.

Three different questions

PropertySimple meaning
HardnessHow well does the surface resist scratches?
ToughnessHow well does the material resist chips and breaks?
StabilityHow does it respond to heat, light, moisture and chemicals?

GIA explains these three parts of gem durability in More than the Mohs Scale.

Why the Mohs scale is only one clue

The scale compares scratch resistance. Its numbers are not evenly spaced. A stone at 10 is not simply twice as hard as one at 5. The number also does not measure resistance to a sudden impact.

Can diamond break?

Yes. Diamond resists scratching extremely well, but a hard blow can chip or split it. Cleavage describes directions in a crystal where it can split more easily. GIA's diamond care guide explains why even diamond needs careful handling.

A practical example: moonstone

Moonstone has a hardness of 6–6.5, but poor toughness. That is why its care involves more than avoiding scratches. Read our moonstone cleaning guide for the separate cleaning steps.

Questions to ask before buying

  • What is the material, and has it been identified?
  • Will I wear it as a ring, pendant or earrings?
  • Are treatments, repairs or existing cracks disclosed?
  • Which cleaning method is suitable for this specific piece?

Keep the answers with your receipt or report. A simple care note is more useful than a claim that a gem is “unbreakable.”

Related guides on Gemology Hub

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