The shell of the ammonite was originally composed of aragonite which becomes unstable under high temperature and pressure. When it is unstable, it reverts to calcite. The process occurs over tens of millions of years.
Similar to the shell of abalone, the ammonite shell's inner surface had layers of mother of pearl. This is actually the layers of aragonite sandwiched in between proteins. As the shell turns into a fossil, the mother of pearl layer might also transform into ammonite, which is akin to a layer of opal. And who can resist the sacred geometry of an ammonite?
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