Very interesting thread. I'll add my 2 cents as a former physics student who spent a year or two measuring atomic structure with tunable dye lasers.
Some people here compared the sparkles to the speckle patterns caused by interference of coherent laser light. I think those comparisons are right. The light we're working with here is not laser light, but the same speckle patterns arise any time the light source looks like a point (from the perspective of the subject).
They can be seen by your naked eye on your own thumbnail in direct sunlight on a very clear day. They can also be seen on your thumbnail at night when you walk under an older metal-halide streetlight that is not too diffused. The patterns are especially obvious in the light cast by carbon-arc lamps or welding torches.
Once you start looking, you'll see them everywhere!
False color and detail/DOF from shiny metal
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