Limestone is a sedimentary rock consisting of more than 50% calcium carbonate (calcite - CaCO3). There are many different types of limestone formed through a variety of processes. Limestone can be precipitated from water (non-clastic, chemical or inorganic limestone), secreted by marine organisms such as algae and coral (biochemical limestone), or can form from the shells of dead sea creatures (bioclastic limestone). Some limestones form from the cementation of sand and / or mud by calcite (clastic limestone), and these often have the appearance of sandstone or mudstone. As calcite is the principle mineral component of limestone, it will fizz in dilute hydrochloric acid.
Other specimens - Click the thumbnails to enlarge
Grain size - variable, can consist of clasts of all sizes.
Hardness - generally hard.
Colour - variable, but generally light coloured, grey through yellow.
Clasts - if clastic / bioclastic then grains and / or broken or whole shell fragments visible; if non-clastic / chemical then crystalline and no clasts visible.
Other features - smooth to rough to touch, dependent on composition and / or mode of formation.
Uses - base for cement; as dimension stone for decoration of walls and floors; in the production of lime fertilizer, paper, petrochemicals, pesticide, glass etc.
New Zealand occurrences - widespread occurrence throughout the country; exposed in many quarries, and on some coastlines; notable localities include the Te Kuiti area in the North Island (home of the Waitomo Caves), Pancake Rocks at Punakaiki on the west coast of the South Island, and Trilobite Rock in the Cobb Valley in northwest Nelson.