Laboratory Manual for Introductory Geology 4e

FIGURE 3.1 Specimens of the mineral fluorite showing only some of its colors.
3.4.2 Luster
One of the first things we notice about a mineral is its luster: the way light interacts
with its surface. For mineral identification, we distinguish minerals that have a metallic
luster from those that are nonmetallic. Something with a metallic luster is shiny
and opaque, like an untarnished piece of metal. Materials with a nonmetallic luster are
said to look earthy (dull and powdery), glassy, waxy, silky, or pearly—all terms relating
their luster to familiar materials. Luster is a diagnostic property for many minerals,
but be careful: some minerals may tarnish, and their metallic luster may be dulled.
3.4.3 Color
The color of a mineral is controlled by how the different wavelengths of visible light
are absorbed or reflected by the mineral’s atoms. Color is generally a diagnostic
property for minerals with a metallic luster as well as for some with a nonmetallic luster.
But specimens of some nonmetallic minerals, like the fluorite in Figure 3.1, have
such a wide range of colors that they were once thought to be different minerals. We
now know that the colors are caused by impurities. For example, rose quartz contains
a very small amount of titanium.
3.4.4 Streak
The streak of a mineral is the color of its powder. We don’t crush minerals to identify
them, but instead get a small amount of powder by rubbing them against an
unglazed porcelain streak plate. Streak and color are the same for most minerals, but
for those for which they are not the same, the difference is an important diagnostic
property. A mineral’s color may vary widely, as we saw in Figure 3.1, but its streak is
generally similar for all specimens regardless of their color, as seen in FIGURE 3.2.
58 CHAPTER 3 MINERALS