The Diamond Ring effect occurs just before and just after totality when, for just a few seconds, a very small part of the Sun’s very bright photosphere is still showing at one edge of the Moon, but the inner corona is visible around the rest of it. The combination of these looks like a dimly glowing ring with a very bright spot on one edge – hence the name. (The effect transitions into Baily’s Beads as the Moon covers more and more of the remaining sliver of photosphere.) The image below is one of several frames captured with our C8 telescope setup showing the diamond ring seen just before totality. (The protective solar filter was removed about 10 seconds before totality, enabling us to capture the diamond ring and Baily’s Beads.)
It is interesting to contrast images recorded just before and just after the solar filter was removed, showing how tiny the sliver of visible photosphere is that produces the effect.
Here is a low-resolution version of the movie from which these frames were taken. It starts just before the solar filter was removed and lasts until Baily’s Beads are disappearing.