- calciumThe chemical element (symbol Ca) with atomic number 20: a soft, silvery-white alkaline earth metal which occurs naturally as carbonate in limestone and as silicate in many rocks.
- chalkA soft, white, powdery limestone (calcium carbonate, CaCO₃).
- causewayA road that is raised so as to be above water, marshland, and similar low-lying obstacles, which in some cases may flood periodically (e.g. due to tides). Originally causeways were much like dykes, generally pierced to let water through, whereas many modern causeways are more like bridges or viaducts.
- calciteA very widely distributed crystalline form of calcium carbonate, CaCO₃, found as limestone, chalk and marble.
- calicheA crude form of sodium nitrate from South America; used as a fertilizer.
- calcaneumThe calcaneus.
- calcographydrawing with chalk
- calthemiteAny secondary calcareous deposit, created by the action of water outside the natural cave environment. They are typically found on man made structures and mimic the shapes and forms of cave speleothems, such as stalactites, stalagmites, flowstone etc.
- caltropA metal object, usually small, with spikes arranged so that, when thrown onto the ground, one always faces up as a threat to pedestrians, horses, and vehicles (also used as a heraldic charge).
- calxThe substance which remains after a metal or mineral has been thoroughly burnt, once seen as being the essential substance left after the expulsion of phlogiston, but now recognised as being the metallic oxide (or, in some cases, the metal in a state of sublimation).