Ammolite is composed primarily of aragonite, a form of calcium carbonate (CaCO₃), and belongs to the carbonate mineral group. It formed over 70 million years ago during the Late Cretaceous period when ammonite shells were buried under sediment and gradually mineralized. Its crystal system is orthorhombic. The most striking feature of Ammolite is its play of color, caused by light interference within its layered structure. The color range can include bright reds, greens, blues, and yellows, with some rare specimens showing multiple colors at once. This effect is similar to opal but has a more structured pattern. Ammolite has a hardness of about 3.5 to 4 on the Mohs scale, making it relatively soft and fragile compared to other gemstones. Its specific gravity ranges from 2.6 to 2.9, and its refractive index is approximately 1.52 to 1.68.