Sponges:
Most sponges do not make completely mineralized walls. Instead, they secrete internal skeletal elements called spicules within their tissues that serve to stiffen the sponge body into its characteristic shape. Spicules range from simple rods to complex, bi-, tri- and multi-axis shapes. When sponges die, they release spicules as silt- and sand-size particles. Most sponge spicules are made of diagenetically unstable opal which tends to dissolve or to be replaced by crystalline quartz or calcite.