Color in Minerals
How do we perceive color?
color perceived depends on the light the object is viewed under: the effect
of illumination type can be very important (fluorescent vs. incandescent
rods: low intensity light -> one color perceived (gray)
cones: three pigment types RGB, thus color is seen = %r + %g +%b
eyes most sensitive to green light.
|How do we perceive color?
Why do things look colored?
Causes of color
We see radiation with wavelengths in the "visible"
Visible spectrum: Red, Orange,
Why do things look colored?
thing are colored when some process removes some wavelengths (absorbs
specific wavelengths) from the visible spectrum
Question: blue sapphire viewed in candle looks black, why ?
Blue sapphire is blue because this is the only wavelength range of
visible light that can be transmitted by the stone. Candle light is rich
in red wavelengths and poor in blue wavelengths. Thus, the wavelengths
(colors) of visible light available are exactly those that can NOT be transmitted.
Result: no light is transmitted, the stone appears black!
If you understand this, then you understand some of the important
basic concepts in this module!
Physical processes occurring in the
An electron transition requires a specific
amount of energy, and can only use light with a specific wavelength
(each wavelength having a corresponding energy).
Let's _define_ adsorbtion here?
However, if energy is dissipated in other ways, we see the color of the
light that was NOT adsorbed, e.g., if adsorb red and orange light and the
energy of these wavelengths is lost (e.g., as heat), see Y+G+B+I+V (green-blue).
luminescence: electron returns to its ground state (where it started)
and releases the amount of energy it absorbed (thus returns light with
that wavelength to the spectrum, thus, no color....
fluorescence: If some of the adsorbed energy is lost but the reminder
is returned to the visible spectrum, the light returned has lower energy
and thus, different color. [changes in energy = changes in wavelength =
changes in color].
of energy....View this movie!
Click for larger image!
||dispersed metal ions
band theory (not required for EPS2)
physical optics (covered later)
Impurities cause color in gems!
Impurities are elements (e.g., Ti, V, Cr, Mn, Fe, Co,
Ni, Cu...) that are not present in the pure compound. Impurities are elements
that occur in low concentration in the gemstone.
A ruby may contain < 1% Cr and it will look pink or red, but
the same material without Cr will be completely colorless. This example
contrasts with gems such as turqoise, in which the color-causing impurity
is a major ingredient.
If we take one mineral, beryl,
and add different impurities, we get different colors:
Beryl containing iron (Fe):
Beryl containing Manganese(Mn):
Aquamarine = Fe++, beryl is blue
Heliodor = Fe+++, yellow
Green beryl : due to mixtures of Fe2+ and Fe3+
Beryl containing Chromium(Cr):
From the above examples it is clear that the oxidation state (e.g.,
Fe2+ vs. Fe3+) also affects the color!
Morganite : Mn++ is pink
Red beryl : Mn+++ is red
If impurity ions produce color, the color can be changed if the oxidation
state can be changed.
Note: heating beryl that is green or yellow reduces ferric iron, and
the beryl turns blue. This greatly increases the stone's value.
The process of color change can simply involve heating the stone in
a low oxygen atmosphere. This could be done by wrapping the stone in paper
and allowing the paper to burn
Mn+++ is efficient at absorbing light, (blue end of the spectrum)
thus color is strong.
The same impurity colors different gems differently!
Chromium (Cr+++) in ruby: red
Chromium (Cr+++) in beryl: emerald green
Chromium(Cr+++) in alexandrite: purplish or red (see below!)
This effect is because the Cr absorbs light differently when it
is in beryl, emerald, and alexandrite. This is illustrated here for ruby,
alexandrite and emerald
Note the different regions of absorption and transmission in the above
In the case of ruby, the largest valley (transmission window = low in
the absorption graph) occurs at the red end of the spectrum, thus the stone
essentially looks red. (However, a smaller transmission window may occur
at blue wavelengths (as shown). This gives a purplish cast to the red color
In the case of emerald, most tramsmission occurs at green wavelengths
and most other wavelengths are absorbed strongly. Thus, emerald looks green.
The "Alexandrite" color change effect:
an example where the details are important! Color change
due to change in the color of incident light! (recall that fluorescent
light is bluish (rich in blue wavelengths) and candle light is rich in
red and orange wavelenghts).
Alexandrite is the best known example of a gemstone that changes
color depending upon the light it is viewed under.
In the case of alexandrite, there are two approximately equal
windows - the first at blue and second at red wavelengths. When viewed
in light made up of all wavelenghts, the stone tramsmits blue and red and
often looks purple or purple-grey.
Here is a diagram showing the:
of illumination of alexandrite with regular (white) light
When viewed in light containing mostly red wavelengths (e.g.,
candle light) the stone looks red. This is understood because, although
the stone could transmit blue light, there is no blue light to transmit.
Here is a diagram showing
illumination of alexandrite by reddish light
The reverse is also true. In light rich in blue wavelengths (e.g., fluorescent
light), the stone looks blue because, although it could also transmit red,
there is little red in the light to transmit.
Here is a diagram showing illumination
of alexandrite by light rich in blue wavelenghts. Different specimens
of the same gem will be characterized by slightly different adsorption/transmission
characteristics (different adsorption spectra shapes) and so their colors
Note: this color change effect in response to change in illumination
type (e.g., incandescent vs. fluorescent) is not restricted to alexandrite!
Many gems have color change varieties, e.g., sapphire, garnet.
change garnet viewed in fluorescent light
change garnet viewed in incandescent light!
In all cases the explanation for color change is the same, involving
the range of wavelengths in the light and the ability of the stone to transmit
two different ranges of wavelengths of light (e.g., red and green).
Visit a spectroscopy site
with many additional examples of
color caused by impurities!
Charge Transfer causes color in gems.
Charge transfer can only occur in compounds that have at least
two elements in different and variable oxidation states. Charge transfer
can produce very intense colors in gems and minerals.
The term charge transfer refers to the process where
are swapped between elements. Examples of elements that can participate
in charge transfer are:
A crystal contains metals (M) in two oxidation states: M2+ and M4+
M2+ can loose an electron and become M3+
M4+ can accept the electron (from above) and become M3+.
Thus, the crystal can exist with
M3+ plus M3+ or M2+ plus M4+.
As you can see, these pairs are interchangeable by movement of an electron.
This is described more fully as intervalence charge transfer!
Visit a spectroscopy site with additional information
caused by charge transfer.
cation - cation
sapphire: Fe++ <-> Ti4+ ,
red light therefore...Deep
blue of sapphire
in beryl, Fe++ and Fe+++ exchange of electron (charge transfer): requires
energy = red light, therefore you'll see...aquamarine
; with more Fe+++ -> greener
color due to absorption
In tourmaline, Mn++ <-> Ti 4+, and the result is a yellow-green
anion - anion
(in lapis lazuli) involves charge transfer between a triangle of sulfur
cation - anion :
Color centers are imperfections in crystals that cause
color (defects that cause color by absorption of light).
They are most often due to radiation damage: e.g., damage due to exposure
to gamma rays. This irradiation may be from both natural (U, Th, K in minerals)
or artificial sources. In rare cases, UV light can produce color
If damaged by radioactive decay, electrons can be removed from their
normal sites, bounce around, loose energy, and eventually come to rest
in a vacant site in the structure (a trap).
One crystal may have many different types of electron traps
Electrons in specific traps absorb only a certain range of wavelengths,
color that is seen is the color not absorbed by these trapped electrons.
Because they are a form of damage, color centers can be removed by addition
of energy. This may involve heating the stone to a few 100 C.
Example: Heat treat brown zircon, it may turn blue!!
(this is a common gem treatment!)
Review: when electrons escape
their traps, color centers are
removed, so color is removed.
In some cases, exposure to sunlight (especially UV) provides sufficient
energy to remove the color center! - amethyst is an example.
We will revisit this topic when we discuss topaz, for example!
Because irradiated minerals may have several color centers (several traps
with different energies
required to allow electrons to escape, color can be manipulated by
selective removal of unwanted color centers (controlled heating).
Visit a spectroscopy site with additional examples of color caused by
of color in minerals
Important for EPS2 students: Further explanation
of basic concepts.