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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Basalt Columns: Why Cooling Lava Can Look Like Stone Pillars

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Polygonal basalt columns formed by cooling and cracking lava
AI-generated educational illustration. Conceptual only; not a photograph of an actual specimen or location.

A wall of tall stone columns can look as though someone built it. At places such as Devils Postpile in California, the pattern formed naturally as lava cooled.

Cooling creates cracks

Basalt is a volcanic rock. As a lava flow loses heat and contracts, cracks can divide it into columns. Geologists call these fractures columnar joints.

The cracks develop in relation to the cooling surfaces. This is why many exposed columns look upright, while other examples can lean or curve.

Why so many have six sides

Hexagonal columns are common, but the pattern is not perfectly uniform. Columns can have different numbers of sides. At Devils Postpile, the US Geological Survey reports that about 55 percent have six sides.

That variation is useful to remember. Nature can produce an orderly pattern without making every part identical.

Columns are not single crystals

A large basalt column is a piece of rock separated by fractures. It is not one enormous hexagonal mineral crystal. Confusing a joint pattern with a crystal shape can lead to the wrong identification.

When viewing an exposure, compare the tops of the columns with their long sides. The cross-section shows the polygon pattern most clearly. Observe from a safe, permitted viewpoint, away from loose blocks.

These pillars tell a story of heat loss, contraction, and cracking after a volcanic eruption.

Sources: USGS: Devils Postpile columns; USGS: Columnar jointing and cooling.

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