Large Rainbow Corner Negative Cap - CGSS9

Large corner piece of ammolite negative cap showing the full rainbow, from the Bearpaw Formation of Southern Alberta. Red and orange run along the top and left side, bright green and teal fill the middle, and blue and purple come in toward the bottom right, all in a fine crackle pattern on dark shale.

What is negative cap? When an ammonite shell split from the rock around it, part of the ammolite layer sometimes stayed stuck to the rock instead of the shell. That piece is called negative cap: genuine ammolite on its natural shale, showing the colour from the inside of the shell.