Color, while very obvious, is not always dependable for identification purposes. Although some minerals always have the same color, many have various colors depending on the impurities they contain. If a mineral is drawn over an unglazed white ceramic tile, it will leave a streak of powder which is usually a reliable guide to its true identity. Hematite, an iron ore mineral, will produce a bright red streak.
Pyrite gives a greenish or brownish-black streak.
The luster of a mineral measures the degree to which light is reflected by its surface. The crystal shape is an important clue to the identification of a particular mineral, and mineralogists recognize six different crystal systems. Each is based on the number of axes of symmetry developed. An axis is an imaginary line running through the crystal around which it can be turned to produce the same appearance from more than one side. The Staurolite and Selenite in this collection are good examples of the orthorhombic and monoclinic crystal systems, respectively.