PARIS – Nature yields so many treasures, and the placenticeras ammonite featured in our forthcoming Natural History sale on 30 September surely counts as one such treasure. These stones, with their seductive, iridescent colours, are found exclusively in the southern Alberta region of Canada, and one must physically handle them to appreciate all their sparkling nuances. The particular stone offered in our sale is covered in ammolite, a gemstone that gleams like opal or labradorite. The front shimmers with countless reflections of green and orange, and the back has bluish tints. Whenever the stone is repositioned, it bursts into colour in a way that would have delighted the proponents of Art Nouveau, and the mystery of ammonite derives from this abundance of intertwining colour that captures the eye.

A Beautiful Placenticeras Ammonite, Mezozoic, Cretaceous, Bear Pow Formation, Alberta, Canada. Estimate €45,000 — 55,000.

Today, ammonites are used by a handful of adventurous designers who seek to master the art of fantasy jewellery, but they also appeal to aesthetes across a wider spectrum. Ammonites can be admired as rare jewels, but also as works of art. This one is indefinable; one and many at the same time, and that is its extraordinary secret.

Gabriella Mantegani is a specialist in the Jewellery Department, Sotheby’s Paris.