Septaria often contain tiny crystals of calcite precipitated from the sediment pore solution at the surface of the walls of the cracks and cavities. Sometimes it is also possible to observe in large Boom Clay septaria small stalactites grown inside the open cavities. As stalactites are always slowly deposited drop by drop by gravity-driven water flow under unsaturated conditions it is a sign of a more recent evolution when the clay formation was no longer immersed in the sea. Siderite or pyrite coatings are also occasionally observed on the walls of the cavities present in the septaria, giving rise respectively to a panoply of bright reddish and golden colors with characteristic iridescences.