Menilite is a concretion of silica and calcium carbonate often considered a variety of opal.
It forms in nodules of shale and gypsum, which produces these smooth, bulbous forms. Impurities can alter the color, but they typically form in a bright, chalky white.
It was named in 1795 after the Parisian neighborhood of Ménilmontant where it was first described by the French geologist Jean- Claude Delamétherie.
It has since been found to occur in Spain and Japan as well.
There aren’t many details known about menilite’s formation and it represents an understudied area of geology.
As one of the current limits of known science, Menilite is one of the rarer specimens we’ve been able to feature.
It’s cool and feels unusual, but upon looking at it I wouldn’t think it was anything more than a blob of cement or plaster. It does have the nerdy cool factor of a mineral you don’t see too often, though