3. INTRODUCTION
• GEMS: A natural or artificially produced mineral or
other material that has sufficient beauty and durability
for use as a personal adornment.
• The enchanting properties that exquisite a mineral to
be called a gemstone is its quality of Colour.
• Gemstones are generally characterised by pleasant
colour, Transparency, High Refractive Index, Superior
Hardness and often by their unusual properties such as
Chatoyancy and Iridescence.
• Pigeon-blood-red colour of Ruby,velvety green colour
of Emerald, rich blue colour of Sapphire, pleasant
purple colour of Amethyst, gorgeous blue colour of
Turquoise immediately catches the eye.
4. COLOUR
• One of the most enchanting properties that
qualifies a mineral to be called as a Gemstone is
its colour.
• Coloured substances absorb certain parts of
visible spectrum and the resultant colour is a
combination of unabsorbed part of the spectrum.
• Ruby absorbs a greater part of green, yellow
and violet and transmits red and a small portion
of blue.
5. • Perception of colour also depends on the
sensitivity of the human eye.
• In emerald, along with green some amount
of red is also transmitted. Our eye, however
being more sensitive to green ignores red
colour.
Contd.....
6. CAUSE OF COLOUR
The major causes for colours in gems are:-
Chromophores
Colour centre
Luminescence
Physical optical effects
Inclusions
Alexandrite effect
Pleochroism
Colour zoning
7. CHROMOPHORES
• Presence of certain elements is responsible for colour in
minerals. Such specific colouring elements are called
Chromophores.
• Elements that are responsible for colour are the transition
elements (Cu, Cr, Mn, Fe, Co, Ni and V). These elements are
characterised by partially filled electrons in the inner d (3d
electrons) orbitals.
• Unpaired electrons in these orbitals are responsible for colours.
• Of all the colour causing elements, chromium produces
gorgeous colours in many gemstones. Iron is the most common
colour causing element.
8. • A single chromophoric element can produce
different colours in various minerals. Chromium
that has three unpaired electrons in the 'd'
orbitals resulting in fabulous red colour in
corundum (ruby) and enchanting green colour in
beryl (emerald).
• In the case of diamonds it is not the transition
element, but elements like nitrogen and boron
that replace carbon are responsible for yellow-
green and blue colours respectively. This forms a
separate category for the cause of colour, known
as band gap colours.
• Blue colour of sapphire is explained by a
mechanism of transfer of an electron from one
ion to another when the light falls on it. Such
mechanism is termed as molecular orbitals.
Electron transfer or charge transfer can take
place over longer distances.
Contd...
9. COLOUR CENTRES
• Next to chromophores the second major cause for
colouration is structural defects which are mainly
produced by the surrounding radioactive minerals or
such elements present within the mineral in solid
solution. An electron may get displaced by radiation.
• The displaced electron occupies a different position.
The unpaired electron are prone to absorb light
energy and thereby create colours. Such excitable
energy configurations are called colour centres.
•Colour centres are one of the few colouring
mechanisms that can be removed by heating or
exposing the mineral to strong light.
The familiar examples are amethyst, green diamonds
and brown topaz.
Amethyst
10. Gem Formula Colour Origin of colour
Ruby Al2 O3 Red
Cr3+ Replacing Al3+ in octahedral
sites
Emerald Be3Al2(SiO3)6 Green
Cr3+ Replacing Al3+ in octahedral
sites
Alexandrite Al2BeO4 Red/Green
Cr3+ Replacing Al3+ in octahedral
sites
Sapphire Al2O3 Blue
Intervalence transition between
Fe2+ and Ti4+ replacing Al3+ in
adjacent octahedral sites
Diamond C
Colourless,Pale
blue or yellow
Colour centres from nitrogen
atoms trapped in crystal.
11. Band Gap
• Band gap colors are produced in
insulating and semiconducting
materials.
• They require an energy gap between
the valence and conduction energy in
the electronic structure of an atom.
• If the energy band includes all
wavelengths of light the material is white
or clear and an insulator.
• If the band includes the energies of part
of the visible spectrum the material is a
semiconductor and colored.
• Some insulators can be band-gap
colored by impurities.
Eg. diamonds, cinnabar, and cuprite.
12. LUMINESCENCE
• Luminescence deals with production of
colour or visible light in darkness under
certain circumstances. Various
luminescence are observed in minerals.
• Certain minerals emit visible light when
they are exposed to invisible short wave
radiation such as ultraviolet , X-ray or
cathode rays. This phenomenon is
called fluorescence.Eg:-fluorite. If it
continues to glow even after these rays
are cut off, it is called phosphorescence.
• Gemstones like ruby produce bright red
flourescence and Australian white opal
and certain diamonds show fluorescence
as well as prolonged phosphorescence.
13. PHYSICAL OPTICAL EFFECTS
• Colour can also be caused due to certain
physical optical effects such as iridescence and
dispersion.
• Interference colours are caused when light
travels obliquely through materials with thin
layers of differing refractive index.
• The layers have to be about as thick as a
wavelength. A coherant ray of light shining
through the material is dispersed in the new
medium.
• At each interface some of the light is reflected
back up and some continues on down.
14. contd…..
• Scattering of light from minute inclusions
produce a bluish to milky white iridescence
called opalescence in opal.
• In chatoyancy, scattering of light from
inclusions of rutile needles and asbestos
fibres give rise to brilliant silky reflections in
star corundums and tiger eye respectively.
15. SCATTERING
• Scattering of light can cause colors to appear
because blue light is more prone to scattering
than red.
• Scattering is caused by submicroscopic (the
finer the better) grains of solid or liquid,
material.
• It can even be caused by random collisions of
gas molecules in the atmosphere.
• Scattering is responsible for the blue of the
sky, the white of clouds and the aurora red
color of sunsets.
• Minerals that display a special case of
scattering are moonstones, cat’s eyes, and
asterated (star) stones.
16. INCLUSIONS
• One reason for colour in gems is on account
of inclusions of unconcerned materials.
• Example: inclusions of haematite flakes in
aventurine feldspar(sunstone)produce bright
orange red reflections, called
aventurescence.
• Aventurine quartz with various inclusions like
iron oxides, actinolite and rutile is used for
carving.
• Minerals with crystallographically oriented
inclusions are cut in cabochon fashion to
obtain cat's eye effect and asterism.
17. Alexandrite effect
• The term alexandrite effect refers to the apparent change of
color in certain minerals from blue-green or greenish violet in
daylight to red or reddish violet in incandescent light.
• The origin of this color change is often attributed to dichroism,
but simple observation shows that the change depends chiefly
on the nature of the incident light.
• Absorption spectrum of all alexandrite-like minerals is
characterised by transmission maxima in the blue-green and
red regions and by a transmission minimum in the yellow
region.
• Careful study of the color changes in chromium compounds
by means of tristimulus analysis proves that the color change
is due to the response of the human eye and brain and not to
any unexpected changes in the properties of the stone.
18. Pleochroism
• The property of certain minerals exhibiting different
colors when viewed from different directions under
transmitted light in polarised light.
• In tourmaline and iolite difference in absorption is so
strong that pleochroism is observed without
polarised light.
• Certain tourmaline absorb pleasant green colour
when viewed along the length and right angles to it
i.e. when viewed through the basal plane, it absorb
deep brown or it may even black.
19. Colour Zoning
• Colour zoning is the result of variation of
the mineralising fluid or supply of
impurities at the time of mineral growth.
• Colour zoning may be concentric,
longitudinal, irregular, or patchy. Eg.
Amethyst, corundum, tourmaline etc.
• Colour zoning becomes obvious when the
stone is immersed in high refractive index
liquid or even in water.
20. Conclusions
• The most important qualities of
gemstones are colour, brilliance,
durability and rarity.
• In physical optical effect, interference
colours are caused when light travels
obliquely through materials with thin
layers of differing refractive index.
• Among the transition elements (Cu, Cr,
Mn, Fe, Co, Ni and V), iron is the most
common colour causing element.
21. REFERENCES
• Borner, Rudolf, 1962. Minerals, Rocks and
Gemstones. London, U.K.
• http://en.wikipedia.org/wiki/gems.
• www.gems and gemstones.
• Gems and Gem Industry in India.By
R.V.Karanth.
• Prasad, Umeshwar, 2000. Economic Geology,
Patna, India.
