Glossary of terms

 Gypsum: A widely distributed mineral consisting of hydrous calcium sulfate (CaSO₄.2H₂O), which is soft (hardness of 2 on the Mohs scale), white or colourless when pure, but commonly has tints of grey, red, yellow, blue or brown. It is either massive (alabaster), fibrous (satin spar) or in the form of monoclinic crystals.  It is used chiefly as a soil amendment, as a retarder in portland cement and in making plaster of Paris. 

 Mohs Scale: A standard of ten minerals by which the hardness of a mineral may be rated. The scale runs from the softest to the hardest and is numbered one to ten: talc; gypsum; calcite; fluorite, apatite; orthoclase; quartz; topaz; corundum; and diamond.

 Retarder: A substance that when added to cement or gypsum prolongs the setting time of the plaster.

Portland (cement): A hydraulic cement made from naturally occurring limestone containing up to 25% argillaceous material.   

 Argillaceous – Either resembles, is largely made of, or contains clay size particles or clay minerals. 

  Igneous activity: Indicates the magma activity in the Earth that forms rock or mineral.

Metamorphism: The mineralogical, chemical and structural adjustment of solid rocks to physical and chemical conditions which have been imposed at a depth below the surface zones of weathering and cementation, and which differ from the conditions under which the rocks in question originated.

UNESCO: United Nations Educational, Scientific, and Cultural Organisation.