Minerals occur under a variety of circumstances. It is important to understand their occurrence as the type of formation in which they occur determines the relative ease and cost of extraction of said minerals. In igneous and metamorphic rocks, minerals in their molten or gaseous forms are often pushed nearer to the earth’s surface through crevices. They cool and solidify in these crevices. The smaller occurrences are called veins while the comparatively larger ones are called lodes. Major metallic minerals like tin and copper occur in nature in this manner.