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Many properties that help identify a mineral
are related to light. Luster, is the way that a mineral reflects
light. There are two types of luster. Metallic minerals look like shiny
or rusted metal. Nonmetallic elements reflect light like glass, pearls,
or glue. The color of a mineral is sometimes very helpful. For many
minerals with a metallic luster, color is an important means of
identification. The lead gray of galena, the brass yellow of pyrite, or
the blue of azurite are very diagnostic. However, many times the color
may tarnish so it is important to look at a fresh surface.
The color of a fine powder of a mineral is a known
as its streak. You can use a porcelain plate to scratch a
mineral to observe its streak. Hematite has a characteristic red-brown
streak, no matter what form hematite comes in.
The refraction of light is important in a
few minerals, especially calcite. It produces an image that is doubled.
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