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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Albite Mineral: Properties, Identification and Collector Care

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A–Z Minerals: A — Albite. A pale crystal beside another mineral may look unimportant. Albite is worth a closer look. It is a common feldspar that helps us understand many rocks.

White albite pericline crystals from the Ziller valley in Austria
Albite (pericline), Ziller valley, Austria; specimen about 10 × 5 cm. Photo: Didier Descouens, CC BY-SA 4.0, via Wikimedia Commons. Image unchanged.

What is albite?

Albite is the sodium-rich end member of the plagioclase feldspar series. Its ideal formula is NaAlSi3O8. It has a triclinic crystal structure. In simple terms, this describes how its atoms are arranged.

Albite properties at a glance

  • Hardness: 6–6.5 on the Mohs scale.
  • Color: commonly white or gray; other pale colors occur.
  • Streak: white.
  • Luster: glassy, sometimes pearly on cleavage surfaces.
  • Specific gravity: about 2.60–2.65.
  • Breakage: brittle, with strong cleavage directions.

Where does it form?

Albite occurs in granite, pegmatites and several other rocks. It also grows in some mineral veins. Quartz and mica may occur with it. Its name comes from a Latin word meaning white. These facts are documented in the Handbook of Mineralogy albite entry.

How to examine an albite specimen

Use a hand lens and side lighting. Look for flat crystal faces and fine parallel lines caused by twinning. Take photographs before you move a fragile cluster. Color alone cannot confirm albite; several feldspars look similar. For a firm identification, ask a mineral specialist about suitable testing.

Collector care

Support the base instead of pulling a crystal. Do not scratch a display specimen to prove its hardness. Before cleaning, identify every mineral on the piece, including the base rock and any repairs. For an unfamiliar mixed specimen, keep handling gentle and ask for specimen-specific advice rather than starting with acids or vigorous brushing.

A useful collection note

Record the mineral name, locality and seller or collector information. If the identification is tentative, write “possible albite” instead of turning a guess into a fact.

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