Menilite Opal from Spain in matrix

This is a very strange and rather rare mineral: the Ménilite Opal. Initially discovered and named in Ménilmontant, Paris. Discovered as nodules in shale, Menilite Formation? in 1795. But this group comes from southern Spain: Agramon, near Murcia.
Spanish "menilitas" are found in a field of diatomaceous earth (fossils of marine microorganisms with "glass" skeleton -silica-). The nodules are made of massive opal sometimes light to light-grey caramel, with abundant diatomaceous inclusions (themselves composed of opal), with a chalky white surface of powdery opal, so three "varieties" of opal in a single nodule. The interiors are hard and compact and take a good varnish, and are sometimes used for opaque lapidary work.