Stone Type: Chert
Chert is a sedimentary rock composed of microcrystalline quartz. It usually formed by the precipitation of silica into other sedimentary rocks, occurring as nodules or layers in limestone. It forms when tiny organisms extract pure silica from ocean water and use it as body armour or skeletons. When the organisms die, their skeletons dissolve and form a siliceous ooze, which eventually precipitates, coalesces, and recrystallises as the surrounding limestone rock is formed.
Marine cherts sometimes contain fossils of organisms like diatoms and radiolarians, as well as sponge spicules. Petrified wood is a form of chert where the silica has replaced the structure of the fossil. The internal structure of chert is amorphous and lacks layering, which makes it ideal for stone-flaking. It occurs in a wide variety of colours, and most varieties respond well to heat-treatment to improve the material’s flaking characteristics. Chert is a general category which includes folk classifications like flint, chalcedony, opal, novaculite, porcellanite, and jasper, and many archaeologists prefer to use the folk classifications when describing artefacts.