These may be siliceous deposits or calcareous deposits. The cementing material may be siliceous or argillaceous and is sometimes calcareous. Palynology does not include diatoms, foraminiferans or other organisms with siliceous or calcareous exoskeletons. It also prefers to grow in siliceous, calcareous, sandy, alluvial and clay soils. The poriferan 'skeleton' consists of microscopic calcareous or siliceous spicules or a spongin network. Sponges can be calcareous, siliceous, or composed of spongin. Boulder opal consists of concretions and fracture fillings in a dark siliceous ironstone matrix. It can be derived from naturally occurring limestone and diatomaceous earth, a siliceous sedimentary rock. Diatoms, radiolaria, and siliceous sponges use biogenic silica as a structural material for their skeletons. The remains of radiolarians are preserved in chert; a byproduct of siliceous ooze transformation. Lake Magadi is also well known for its extensive deposits of siliceous chert. This polygonal tessellation is best developed in relatively fine-grained, uniform, and siliceous or silicified sandstones. Kastania is located on one of the relatively few sedimentary or siliceous outcrops. Gold mineralization was discovered with pyrite in slightly siliceous tuff. Skeleton fragments from siliceous organisms are subject to recrystallization and cementation.