Other types of Onyx stones are green stones that are mostly used in religious places in Iran. If there is some dolomite in the limestone, the magnesium in the dolomite mixes with other materials under the action of metamorphism to form a green Onyx . Onyx is resistant to adverse conditions such as heat and cold and sunlight and rain. Onyx stones are obtained from the transformation of limestone. Its most important mineral is called calcite. When limestone is subjected to high pressure and heat for a long time, it transforms and eventually turns into Onyx . In places where there are hot springs, there is also Onyx and travertine.