Natural rubber [(CH2CHCCH3CH2)] occurs in a large variety of plants. It is amorphous when unstretched, while a crystalline structure appears on stretching or at low temperature. At room temperature it has a specific gravity of 0.93, although this value can vary slightly depending on any impurities present. Natural rubber is soluble in most organic compounds and is easily oxidized. The process of vulcanization, sulfur cross-linkages, improves its tensile and thermal characteristics.