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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Andalusite Mineral: Properties, Crystal Shape and Formation

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A–Z Minerals: A — Andalusite. Andalusite is an interesting mineral for both geology students and crystal collectors. Its shape and the rocks around it can tell a useful story.

Andalusite mineral specimen
An andalusite specimen. Photo: Moha112100, CC BY-SA 3.0, via Wikimedia Commons. Image unchanged.

What is andalusite?

Its formula is Al2SiO5. Kyanite and sillimanite have the same chemical formula, but their atoms are arranged differently. Minerals like these are called polymorphs.

Simple andalusite properties

  • Crystal system: orthorhombic.
  • Hardness: about 6.5–7.5 on the Mohs scale.
  • Specific gravity: about 3.13–3.16.
  • Streak: white.
  • Shine: glassy.
  • Shape: crystals may be long, with a nearly square cross section.

Pink, green, gray and other colors occur. A crystal may also contain many inclusions, so transparency varies. The Handbook of Mineralogy gives the mineral data and occurrence details.

How does andalusite form?

It commonly forms when clay-rich rocks are changed by heat near an intrusion of molten rock. This is called contact metamorphism. It also occurs in some other metamorphic rocks. The mineral can therefore help geologists study how a rock changed.

What should a beginner look for?

Start with the whole specimen. Note its crystal shape, surrounding rock and any old label. Then examine individual faces with a hand lens. A square-looking crystal is a clue, not proof of identity. Similar shapes can occur in other minerals.

How to handle it

Andalusite is brittle and has cleavage. Hardness tells you about scratching; it does not mean a crystal cannot break. Support the specimen from below and avoid pressure on a projecting tip. Do not perform scratch tests on a cut gem or an attractive display crystal.

A collector's question

Is every andalusite specimen a gemstone? No. A mineral specimen can be valuable for study even when it is unsuitable for cutting. When buying, ask whether the name was checked and whether the locality is documented.

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