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As implied by the title, each week I will briefly cover a different gemstone to increase my knowledge of the field and hopefully share something interesting with you. Each gemstone has its own neat story to tell, and I have a soft spot for a good story. Posts will usually be on Tuesdays, and the first and third Tuesdays of each month I will talk about that month’s common and less common birthstones. The remaining weeks are my pick (yay!)
Without further ado, this week’s gemstone is ammolite!
Images adapted from http://www.redgallery.de
Ammonites were prehistoric cephalopods that appeared 240 million years ago and became extinct with the dinosaurs 65 million years ago. During their global, aquatic reign they were jet-propelled predators that used sharp beaks and tentacles to catch prey and could be over three feet across. Ammonites were named for the ancient Libyan/Egyptian/Grecian deity Ammon (interesting story there), who was usually depicted with rams’ horns, similar to the shape of their shells. The shell’s chambers follow the Fibonacci sequence (the sum of the two preceding chambers equaling the next), and was originally made of aragonite. Over millions of years in the earth, the shells took on other minerals, including calcite, silica, and pyrite, and trace elements such as copper and iron, becoming ammolite.
The iridescent rainbow of ammolite comes from the microstructure of the aragonite, which makes light bounce around in many tiny layers. Thicker layers produce more reds and greens, while thinner layers produce more blues and violets. Ammolite is graded by the number of colors present in a specimen, and the predominantly blue and violet pieces tend to be more valuable than the reds and greens.
Canada is the main source of ammolite and there are strict laws regarding its mining, including that intact fossils are not eligible for gemcutting. Ammolite tends to be a softer stone (around 4 on the Mohs scale) and so it is normally given a protective layer of quartz or spinel when used in jewelry. As the availability of ammolite is limited, their value is expected to increase with time.
