The phrase "to form a diamond" means to arrange things or people in the shape of a diamond, which is a four-sided figure with pointed edges like a precious gem. Full definition
Grab a dumbbell by placing your palms against the inside of the plates at one end, positioning your hands together so that your index fingers and thumbs form a diamond shape. (bodybuilding-wizard.com)
Any condensed graphite would change into diamond under the high pressures inside the carbon planets, potentially forming diamond layers inside the planets many miles thick. (astrobio.net)
Get into a modified plank position with your knees on the ground, and form a diamond shape with your thumbs and pointer fingers. (health.com)