What is this picture?

Question

What is shown in this image?

Accepted Answer

feldspar cleavage surface. Feldspar cleavage surfaces are so smooth and reflective that they were actually used as mirror substitutes in some cultures before glass became widely available, and you can still see this "schiller" effect today when light bounces off certain feldspars at just the right angle, creating an almost metallic sheen. This natural mirror quality is why some feldspar varieties like labradorite seem to glow with hidden colors from within.

Answer: feldspar cleavage surface