Color is the most eye-catching feature of many minerals. Some minerals will
always have a similar color, such as Gold, whereas some minerals, such as
Quartz
and Calcite, come in all colors. The presence and intensity of certain elements
will determines a specific mineral's color. Minerals with an inherent color (i.e. all specimens of the
mineral are the same color) have essential elements in them which cause their color. Good
examples are Azurite and Malachite, which have their strong
blue and green color due to their copper in their atomic structure. But there are many
minerals which have slight additions of color-causing elements in some specimens that
cause it to be a different color. For example, pure Quartz (SiO2), is colorless,
whereas Amethyst, a purple variety of quartz, has its purple color caused by traces of
the element iron. The amount of iron present determines the intensity of the color.
Certain minerals exhibit a color change when exposed to
light, heat, radiation, or when atomic anominalies are present.
Red Realgar transforms into yellow Paraealgar upon repeated exposure to light. Some minerals, such as Proustite and
Vivianite,
darken upon prolonged exposure to light, whereas other minerals, such as Kunzite fade. Some
minerals undergo color changes when put under intense heat. This method is commonly used
in the gemstone industry to artificially enhance the color of many gemstones.
For example, some varieties of Topaz, Beryl, and Corundum are heat treated to produce
deep colored gemstones from duller stones. Radioactivity can also have an effect, as
is the cause of the color of Smoky Quartz.
Most secondary copper minerals show a bright blue or green color. Iron is usually responsible
for dark red or brown colors, manganese and cobalt for pink, and chromium for deep green. Some minerals,
such as Cassiterite and Zincite, have a chemical structure that would cause them
to be colorless if pure, but due to impurities that are always present, they
are never found colorless. Most secondary uranium minerals exhibit bright neon yellow or green colors.
Often, a mineral has a coating or has been pseudomorphed, causing it to exhibit
the color of the replaced or coated mineral. Two common examples are a Hematite coating
Quartz, which causes the Quartz to be red, and a Limonite pseudomorph after
Pyrite.
Minerals composed mostly of the elements aluminum, sodium, and
potassium are usually colorless or very lightly colored. In some cases, the color of a
mineral may depend on its atomic bonding rather than composition, such as by
Diamond and Graphite. Both these minerals have
the same are formed from the same material (carbon), yet one is almost always white or
very lightly colored, while the other is dark gray to black. Anominalies in the chemical structure
of Halite are responsible for the deep blue and purple hues sometimes seen in this otherwise lightly
colored mineral.
Inclusions of one mineral within a host mineral can also cause a
color difference. Two fine examples are:
1) Chlorite inclusions in Quartz, causing the otherwise clear Quartz to be green.
2) Rutile inclusions in Quartz, which give the Quartz a golden hue.
Several minerals tarnish or oxidize, thereby affecting their color. The
best examples are:
1) Silver, which tarnishes black, yellow, or brown.
2) Copper, which oxidizes green.
3) Bornite and Chalcopyrite, which oxidizes an iridescent array of colors.
Some minerals, such as Opal, display a multicolored
effect when viewed from different angles. This is called opalescence, or "Play of Color". A few minerals appear to
change color when viewed in different light. Alexandrite, a variety of Chrysoberyl, is
usually dark green in
natural light, but takes on a purplish hue in artificial light. Other minerals will change
color when viewed at different angles. This is called dichroism or pleochroism.
Cordierite, the most famous
dichroic mineral is bluish-purple, but turns gray when rotated or viewed at a different
angle. The properties of opalescence, labradorescence, and dichroism, as well as other optical properties are explained in greater
detail in the section "Other Properties".
Mineral colors may be artificially enhanced in various
ways, especially when they are used as gemstones. This includes heating and irradiation
(both previously mentioned), as well as dying. A few minerals, especially Agate, are
sometimes
dyed to enhance their color.
How to use color as an identification
mark
Color alone will not normally identify a mineral, but it
can be a very helpful factor. Many minerals come in a diversity of colors, and many
different minerals have similar colors. However, a few minerals can be identified solely
by their color (usually in addition to crystal shape), for example Azurite,
Rhodochrosite, and Dioptase.
Such minerals are known as monochromatic minerals.
Together with color, a crystal shape can identify most of the
better known minerals. It is best to observe the mineral in good lighting, preferably
daylight. Make sure not to observe a tarnished
or discolored surface.
