To test hardness, see if the mineral can scratch common materials like fingernails, pennies, nails, glass, etc. When a drop of HCl touches calcite, carbon dioxide gas bubbles off from the chemical reaction between calcium carbonate and hydrochloric acid. Do not confuse with faces formed by breakage along cleavage lines. Test using material such as paper clips or compasses . Rocks are aggregates of minerals, and although two thousand different minerals are known, only a few very common minerals make up most of the rocks in our geological environment.
Some minerals are essential industrial resources.
Others contribute to environmental pollution. Rocks are classified on a basis of origin, geologic history, and composition. Igneous Rocks are cooled from liquid magma . Those that cool slowly within the earth have large crystals and are called intrusive . Those rocks that cool quickly on the surface have smaller crystals and are called extrusive .