I had no idea what it was either until I looked it up haha. But I have heard of aragonite! So I found this really interesting
Ammolite: Meanings, Properties and Powers
Ammolite Properties, Meanings, Powers & Uses
Healing Properties of Ammonite and Ammolite
“Stone of Structure & Stability: There has been much confusion between Ammonite and Ammolite in the metaphysical world, often falling on the side of describing the Ammolite and not the Ammonite. Ammonite is not a gemstone, but rather a fossil. It’s name was inspired by the spiral shaped of the fossilized shells, being that of a tightly coiled ram’s horn. Pliny the Elder (circa 79 AD) referenced this name for the Egyptian God Ammon – who was typically depicted wearing rams horns. It must also not to be confused with a Nautilus, which has a similar shape but an entirely separate make up. Ammonites are an extinct group of marine animals. The words ammonite and ammonoid are both used quite loosely in common parlance to refer to any member of subclass Ammonoidea. However, in stricter usage the term Ammonite is reserved for members of suborder Ammonitina (or sometimes even order Ammonitida). Ammolite on the other hand, is an opalized version of the Ammonite; found primarily along the eastern side of the Rocky Mountains from the US and Canada. These are the fossilized shells of the Ammonite; the opalization is formed by minerals filling in the sedimentary rocks or veins in rocks; it has also been known to replace organic materials in fossils, wood and shells, even bone. The ‘opalilzed’ or iridescent Ammolites are comprised primarily of Aragonite, which is the same mineral that creates a nacreous Pearl. Ammolite was given the official gemstone status in 1981 by the World Jewelry Confederation. This iridescence would not have been visible during the animal’s life, it is simply the coating or replacing of the shell by the opal. In the Middle Ages Ammolite was known as Draconites; due to their bright colors and what was then considered a bizarre appearance, they were thought be ‘stones’ stolen from a dragon’s head. More recently they have been called Snakestones as well as St. Hilda Stones.”
The book didn’t give any substitutes but I would say ammonite, opal, or any root chakra crystals/stones would be good!