In the last seconds before a total solar eclipse the sunlight breaks into a row of separate points, because it is shining through valleys on the Moon's limb.

The beads are named after Francis Baily, who described them in 1836. When only one is left the effect is the diamond ring. They are also a measurement: knowing exactly where the mountains are, from lunar laser altimetry, lets the duration of totality be predicted to a fraction of a second for any given spot.