Think of a faceted gemstone as a special purpose, retroreflecting prism. Gem cutters cut a pavilion at a particular angle so as little light as possible “leaks” out the stone’s back. This will also send the light, colored and dispersed, back to the viewer. We call this returned light gemstone brilliance.

What is a Retroreflector?

A retroreflector returns light to the source’s direction. As a two-dimensional example, consider a billiards shot into a corner. The ball bounces off one cushion, onto another, and leaves the corner, returning to the shooter (neglecting spin, of course). You can easily find three-dimensional examples, too. Just take a close look at a taillight lens or a highway or bicycle reflector. Have you ever noticed they’re made of arrays of cube-corners? You’ve certainly noticed how bright they are at night.

Various NASA missions have left cube-corner arrays on the Moon’s surface. In order to measure continental drift, the Smithsonian Astrophysical Observatory ran installations in New Zealand, Greece, and Brazil. These locations conducted laser ranging by bouncing lasers from these lunar arrays. Even if the angle wasn’t perfect or the array wasn’t level, the laser beam was returned back to the observatory.

Here’s another good…