Fossil Limestone is the classical building stone used during the Byzantium and Ottoman Empire Era for buildings in Istanbul such as Higha Sophia, the Mosque of Suleyman the Magnificent, Byzantium walls surrouding the old city of Istanbul.  Fossil limestone is made up of the accumulation of oyster sea shells over ages. It is very soft when initially unearthed, allowing the eased production of architectural elements such as columns, window sills, vases; however strengthens with time as it looses its innate water molecules.   Fossil limestone is very light in color and is very decorative with its rich fossil structure for both indoor and outdoor usage. 

Limestone is a type of rock made up mostly of calcite, a mineral form of calcium carbonate. Most limestone is grey, but all colours of limestone from white to black have been found.
All limestone are formed when the calcium carbonate crystallises out of solution. It leaves the solution in many ways, and each way produces a different kind of limestone.
Some limestone can be formed by the work of organisms. Many aquatic organisms draw calcium carbonate out of the water and use it to make their shells and bones. The oysters, clams, snails, corals, and sea urchins do this. When the animals die the shells and bones are broken up by waves into shell and coral sand and mud.
Limestone makes an excellent building stone because it can be carved easily.