Color is an important feature in some minerals, such as fluorite, pyrite, and galena. However, many, such as calcite and quartz, have a variety of colors. Color should always be evaluated on a freshly broken surface. Streak is the tendency of many soft minerals to have a characteristic powder when dragged across a porcelain plate. This is useful when a mineral is polymorphic , such as hematite.
Polymorphic minerals have different forms and colors.
Minerals harder than porcelain will not be powdered.