 Freshly prepared samarium has a silvery luster . In air , it slowly oxidizes at room temperature and spontaneously ignites at 150 ° C. Even when stored under mineral oil , samarium gradually oxidizes and develops a grayish @-@ yellow powder of the oxide @-@ hydroxide mixture at the surface . The metallic appearance of a sample can be preserved by sealing it under an inert gas such as argon . 
