The “diamond ring” phenomenon during a total solar eclipse occurs a few seconds before C2 (2nd contact) and then again a few seconds after C3 (3rd contact). The diamond ring consists of the “diamond” — the sun’s photosphere (either disappearing before C2 or re-emerging after C3), as well as the “ring” — the bright inner corona.

This image was taken just after C3 with a Canon 40D and Canon 100-400 zoom lens @ 400 mm, f/8, 1/60 sec, ISO 100.