CAUSE OF COLOUR AND OTHER OPTICAL EFFECTS IN GEMSTONES

Cause Of Colour and Optical Effects In Gemstones

Introduction of Colour and Optical Effects In Gemstones

Welcome to our journey into “Colour and Optical Effects In Gemstones.” We’re diving into the magic of gemstone hues and optical mysteries. From understanding why gemstones have colours to exploring fascinating theories, let’s uncover the beauty and science behind these precious stones.

Table of Contents

What is Colour

We perceive colour due to selective absorption, transmission or reflection of wavelengths of light. Light is an electromagnetic radiation. When the wavelength is within the visible spectrum (approximate from 400 nm to 700 nm), it is known as “visible light”.

These colours can be produced by visible light of a single wavelength and are called pure spectral or monochromatic colours. When white light falls on an object, say blue turquoise, the turquoise absorbs all wavelengths other than the blue, which is reflected back. This reflected light fans on the retina of our eye and the signals are transmitted to the brain, The brain interprets this as the blue colour of turquoise, Similarly, in case of a ruby, when white light passes through the crystal, green and violet wavelengths are absorbed and the remaining light, which is mostly red with some yellow and orange gives the red colour in a ruby, The human eye can distinguish about 10 million different colours. It is essential to observe the stone in a neutral white light to judge the real colour, If the incident light is rich in blue or red wavelengths the actual colour may not be judged.

ColourWavelength
Violet390-450 nm
Blue450-490 nm
Green490-560 nm
Yellow560-590 nm
Orange590-635 nm
Red635-700 nm
Wavelength range of the visible spectrum

The CIE system is a universally accepted standard for colour specification and measurement. The CIE system can be understood by the “Chromaticity diagram” as illustrated. The pure spectral colours are depicted along the horseshoe shaped outer line. Saturated colours are seen along this outer line of the horseshoe, While the central point is white. The line connecting 400 and 700 nm represents various shades of purple and magenta.

The Chromaticity Diagram (The CIE system )
The Chromaticity Diagram (The CIE system )

A mixture of colours on the opposite side of the horseshoe e.g. yellow and blue, in varying proportion will produce the colours shown in between, including white. Similarly, orange and blue-green in the correct proportion can produce white light. If from this white light, we remove the orange component, then blue-green remains.

Complementary pair is a set of colours which, when mixed in correct proportion. produce a neutral colour (white). Colours on opposite Sides of any line passing through the white spot and intersecting at the periphery of the horseshoe are called complementary colours.

However. when we mix complementary colour pigments, we do not obtain white but black or grey. As discussed above, green pigment appears green because it absorbs all light except green. If we mix this with a violet pigment, which absorbs all light except violet, we get black colour or dark grey colour.

The CIE system also includes a set of defined standard illuminates representing an incandescent lamp (2856K), noon sunlight (4874K), average daylight (6774K) and D65 daylight (6500 K).

The Colour Wheel

A colour Wheel helps us to understand the relationships between colours. Primary colours yellow, blue and red form an equilateral triangle. Green, orange and purple are secondary colours, which are obtained by mixing the primary colours in same ratio. When a primary and a secondary colour is mixed, it gives rise to a tertiary colour. Col ours opposite one another are complementary. In case of aquamarine the blue-green colour is seen because of the combination of blue and yellow primary colour. The red of ruby is a combination of red primary colour and purple secondary colour.

The Colour Wheel
The Colour Wheel

Cause of Colour in Gemstones

Rubies and emeralds would be colourless, but for the tiny percentage of chromium oxide found in these gemstones. Similarly, tourmalines display a wide range of colours because of various chemical impurities like iron, manganese, copper or chromium. These impurities are called ‘Colouring agents’ or “chromophores” Transition elements are the most common cause of colour found in gemstones. The transition elements have the ability to absorb energy from the light rays that pass through the crystal, giving rise to the colour in the gemstone. The colour causing transition elements are Vanadium (V), Chromium (Cr), Iron (Fe), Nickel (Ni), Manganese (Mn), Copper (Cu) and Cobalt (Co). The colour we see is due to partial absorption and partial transmission of wavelengths when white light falls on a gemstone.

When white light falls on the stone, the electrons absorb some wavelengths and get excited to a higher energy level known as excited state. The unabsorbed wavelengths pass through the material giving its colour, The colour we see is complementary to the absorbed light or colours, For example, in case of ruby, white light passes through the crystal, green and violet wavelengths are absorbed to excite the electrons from the ground level to the excited level; the remaining light is mostly red with some yellow and orange which are Visible as the components of red in the ruby.

Similarly, an emerald appears green due to the absorption of red and violet wavelengths and transmission of green. Both rubies and emeralds are coloured by chromium impurity but due to the differences in the strength of ligand fields they exhibit different colours. Ruby has a stronger ligand field; hence absorption is mainly in the green/yellow region and resultant colour is red. In emerald, with weaker ligand field, absorption is mainly in the red/ yellow and violet region and resultant colour is green. In some cases, electrons from the excited state fall back to its normal state or ground state resulting in emission of certain wavelengths. This is known as ‘fluorescence’.

Theories of Gemstone Colour Generation

  • Crystal Field Theory
  • Molecular Orbital Theory
  • Band Gap Theory
  • Physical Optics Colour and Optical Effects in Gemstones
  • Colour Change Effect – Alexandrite Effect Gemstones

Crystal Field Theory

Unpaired electrons may cause colour in the gemstones. This section describes idiochromatic and allochromatic colouration and colour centres. The transition metal ions may be present in a gemstone as an integral part of its composition (idiochromatic) or as an impurity (allochromatic).

Idiochromatic

These are minerals in which the colour causing transition elements are present as an integral part of the chemical composition, If these minerals are crushed, the powder will also be coloured.

GemstoneChemical FormulaChromophoreColour
Almandine GarnetFe3Al2(SiO4)3Iron (Fe)Purple, Red
AzuriteCu3(CO3)2(OH)2Copper (Cu)Blue
HematiteFe2O3Iron (Fe)Red to Brown
MalachiteCu3(CO3)2(OH)2Copper (Cu)Green
Peridot(MgFe)2SiO4Iron (Fe)Green
RhodochrositeMnCO3Manganese (Mn)Red to Pink
RhodoniteMnSiO3Manganese (Mn)Red to Pink
Spessartite GarnetMn3Al2Si3O12Manganese (Mn)Orange
TurquoiseCuAl6(PO4)(OH)8 .4H2OCopper (Cu)Blue
Uvarovite garnetCa3Cr2Si3O12Chromium (Cr)Green
Idiochromatic Self Coloured

Allochromatic

These minerals, in their pure form, are colourless. The colour is caused by small amount of colour causing impurities which are not an essential ingredient of the chemical composition. Example: chromium produces red colour in ruby and green in emerald; absence of chromium impurity will lead to colourless variety. Most of these minerals, when crushed, will produce a colourless powder.

GemstoneChemical FormulaChromophoreColour
AlexandriteBeAl2O4Chromium (Cr)Green to Red colour change
AquamarineBe3Al2Si6O18Iron (Fe)Blue/Green
Blue spinel (synthetic)MgAl2O4Cobalt (Co)Blue
ChrysoberylBeAl2O4Iron (Fe)Yellow. Brown
ChrysopraseSiO2Nickel (Ni)Green
Citrine quartzSiO2Iron (Fe)Yellow
EmeraldBe3Al2Si6O18Chromium (Cr) / Vanadium (V)Green
Grossular Garnet
(Tsavorite)
Ca3Al2(SiO4)3Chromium (Cr) / Vanadium (V)Green
JadeiteNaAlSi2O6Iron (Fe)Green/Yellow/Brown
MorganiteBe3Al2Si6O18Manganese (Mn)Pink
Orthoclase feldsparKAlSi3O8Iron (Fe)Green to Blue
Red berylBe3Al2Si6O18Manganese (Mn)Red
Red spinelMgAl2O4Chromium (Cr)Red
RubyAl2O3Chromium (Cr)Red
Spodumene (Kunzite)LiAlSi2O6Manganese (Mn)Violet/Pink
TopazAl2SiO4(F,OH)2Chromium (Cr)Red
Tourmaline(Na,Ca)(Li,Mg,Al)3Al6B3Si6O27(OH,F)Chromium (Cr)
Manganese (Mn)
Iron (Fe)
Green
Red/Pink
Green/Blue
Allochromatic Gemstone

Colour Centres

The colour of gemstones like amethyst, smoky quartz, blue topaz etc. is derived from colour centres. This is due to the defects in the crystal lattice which give rise to defect holes and colour centres. If an electron is missing from its normal position in the crystal structure, it is called a “defect hole” or a “hole colour centre” resulting in absorption of several wavelengths and hence the colour, if an extra electron is present, it is called electron colour centre. Some of the colour centres are stable and lose their colour only on heating, while others are unstable and may fade when exposed to light.

A common example is of Quartz which is silicon dioxide (SiO2) containing silica ions Si4+.Usually such crystals are colourless, most of the natural quartz crystals have Al3+ impurity replacing Si4. If such stone is irradiated (naturally or artificially), one of the paired electrons can be ejected from the original position on an oxygen ion adjacent to the aluminium site. This would leave an unpaired electron, creating a defect hole, which absorbs certain wavelengths of light, thereby producing the smoky colour. The colour produced in this manner is reversible, i.e. if smoky quartz is heated, the electron will come back to its original position and the quartz will become colourless. This smoky colour lightens when the quartz is heated, becoming yellowish-green (lemon quartz) and then colourless at about 4000C. Heating will destroy the defect hole and all the electrons will move to their original positions to make pairs. If Fe3+ (iron) impurity is present in quartz, pale yellow colour is produced due to ligand field. When this quartz is irradiated, amethyst colour is produced. Heating such quartz (amethyst) produces the yellow colour of Citrine.

Quarts structure
Quarts structure
Distributing a Charge
Distributing a Charge

However, the property of change of colour on irradiation and reverting to original colour on heating is not necessarily an evidence of colour centre. Many gemstones show this property due to irradiation induced change in the valence states of the Chromophores. For example, in aquamarine, Fe2+ produces blue colour that is stable to heat while Fe3+ produces a yellow colouration, when both blue and yellow components are present. it results in green colour. On heating, Fe3+ converts to Fe2+ and thus the yellow component changes to colourless while blue component of aquamarine remains unchanged; this can be reversed by irradiation, these changes do not involve colour centres. Unstable defect holes and colour centres can also be destroyed on exposure to light, irradiated yellow sapphire is one common example which fades off within hours or days on exposure to light. Such colour centres or defect holes are termed as ‘fading colour centres’, Colour centre that change their only on heating are termed as ‘stable colour centres’, like in case of smoky quartz.

GemstoneColour
Stable colour centres
AmethystPurple, Violet
Smoky quartzBrown, Grey
Diamond (irradiated)Greene Blue, Yellow, Brown etc.
Topaz (irradiated)Blue
FluoritePurple
Fading colour centres
Maxixe berylBlue to Green
Sapphire (irradiated)Yellow
Spodumene (irradiated kunzite)Green
Gemstone Coloured by defect holes

Molecular Orbital Theory

Charge Transfer

When an electron is transferred from one transition element to another, it causes absorption of light, and the process is called inter-valence Charge transfer (IVCT). intervalence charge transfer can result in intense colours, as in the case of blue sapphire, The blue colour in sapphire is produced by charge transfer between Fe2+ (iron) and Ti4+ (titanium) ions present at the nearby aluminium (Al3+) sites. One electron from Fe2+ hops to Ti4+, making both trivalent, i.e. Fe3+ and Ti3+; this can be expressed as:

Fe2+ + Ti4+ → Fe3+ + Ti3+

In the process, a certain amount of energy is required, and therefore some wavelengths of light are absorbed, In the above case. most of the wavelengths are absorbed except blue and violet, and as a result, the sapphire appears blue or violet-blue. A deep blue colour results from just a few hundredths of a percent of the required impurities, i.e. titanium (Ti).

The charge transfer may take place between two metallic ions, non-metal to metal, or even two non-metals.

GemstoneColourElements InvolvedType of Charge Transfer
IoliteBlueFe2+ + Fe3+Metal to Metal (same metal)
KyaniteBlue-GreenFe2+ + Ti4+Metal to Metal (different metal)
Lapis lazuliBlueS3+Anion-Anion
SapphireBlueFe2+ + Ti4+Metal to Metal (different metal)
TourmalineBlue and GreenFe2+ + Fe3+Metal to Metal (Same metal)
Gemstone colour due to charge transfer

Band Gap Theory

The term “band gap” refers to the energy difference between the top of valence band (the highest range of energy where electrons are normally present) and the bottom of the conduction band (the range of energy where the electrons get excited and accelerate), Electrons jump from one band to the other by absorbing some energy and thereby produce colour.

This mainly happens in metals or semi-conductors. Only a few stones are coloured by this mechanism like diamond and galena. In diamond the presence of nitrogen or boron gives colour and forms different types of band gaps. The colour of such a gem material depends on the Size of the band gap. If the band gap is large, then no wavelength of light in the visible spectrum is absorbed (everything is transmitted), hence a stone will appear colourless. However, if band gap lies within the visible range, some of the wavelengths will be absorbed While some transmitted, resulting in colour- In case of diamond, nitrogen acts as a donor and absorbs violet or blue wavelengths and hence appears yellow, while boron acts as an acceptor and absorbs red/yellow wavelengths and results in blue colour.

Band gap in diamond
Band gap in diamond

Physical Optics-Colour and Optical Effects

The interaction of light with the physical structure of stone may give rise to various optical effects like dispersion, scattering, diffraction, thin film interference, etc.

Scattering of light

The blue colour of sky is the result of scattering of light by fine particles (dust or gases) in the atmosphere. Shorter wavelengths of light (violet and blue) are scattered more than the longer wavelengths (red/yellow). As the Shorter wavelength (blue) is scattered, the sky appears blue during the day. During sunrise and sunset most of the light strikes tangential to the earth’s surface. Therefore, the path of light through the atmosphere is long and most of the blue and green light is scattered removing blue light from the direct path of the observer. The remaining light is mostly of longer wavelength and thus the sky and the clouds appear red. This also happens during dust storms. As more dust particles get suspended in the atmosphere, more rays get scattered and as a result the sky appears yellow or orange or red, if there was no atmosphere on earth, the sky would be black even during the day, (like on moon or some planets without atmosphere) as there would be no particles to scatter light. In gemstones. the best example of scattering is that of moonstone feldspar. The blue sheen or “adularescence” seen on the surface of the moonstone is the result of scattering of light from minute particles of albite in the layered structure. This scattering, in conjunction with interference from the layers, gives rise to blue sheen.

moonstone a)	Scattering of ligh
Moonstone

Reflection Of Light

Cat’s eye effect (chatoyancy) and Asterism: If the gemstone has a series of microscopic needle-like parallel inclusions and the gemstone is cut as cabochon, it shows a single bright ray of light running across the stone. This ray appears to move with the movement of the source of light or the stone itself. The stone is cut such that the needles are parallel to the base of the cabochon. The chatoyancy effect is due to the reflection of light from the needle like inclusions. The gemstones known to exhibit chatoyancy are quartz, tourmaline, apatite, diopside, chrysoberyl etc.

If the fine needle like inclusions are parallel to more than one crystal face, a star effect or asterism is produced when the gemstone is cut as cabochon with the base parallel to the plane containing the needles, Asterism may be seen in ruby, sapphire, rose quartz, garnet etc.

cats eye Reflection Of Light
Cat’s Eye

Interference

The iridescent colours exhibited by a thin film of oil floating on water are caused by interference of light. Rays of light which strike the surface of a gem are partially reflected back and partially refracted into the gem. If a gem has a layered structure, the refracted rays will again reflect back from the layers beneath the top surface. Since, some rays have travelled a longer distance, they may be out of phase with the other ones, resulting in interference between the reflected rays from the surface and from within the stone. Such interference will give rise to colours as seen in fire agate. along cleavage planes, or in some coated gemstones, A common gem material which exhibits interference is labradorite feldspar. commonly referred to as “rainbow moonstone”. It is known for its bright spectral coloured flashes on the surface. Labradorite is composed of parallel layers of many twin lamellae of varying and alternating composition giving rise to layers of alternating optical densities, Iridescent colours are produced by interference in these layers if the arrangement is regular, uniform and fine enough to produce interference

Reflection
Interference of light

Diffraction

Diffraction is the phenomenon of bending of light around small objects of tiny openings, The most Striking example is that of a compact disc or DVD which has fine grooves. The closely packed groves act as a diffraction grating to produce the bright spectral rainbow colours. Opal is a prime example of a gemstone which exhibits brilliant “play of colour” due to diffraction. Opal consists of a three-dimensional close packed structure of spheres of silica. These spheres form a three-dimensional grating and cause diffraction of light which results in “play of colour”.

Play of Colour
Play of Colour in Opal

Colour Change Effect – Alexandrite Effect

The colour-change effect, commonly known as “alexandrite effect” refers to the change in the body colour of the stone when observed in different types of light sources. typically, fluorescent daylight and incandescent light. In case of alexandrite, colour typically changes from green in daylight to red in incandescent light.

Chromium impurity produces the red colour in ruby and the green in emerald due to the differences in the strength of their ligand-fields. In alexandrite, the ligand field is intermediate between ruby and emerald, which causes the colour change effect. The colour change effect is produced when the transmission band in the red part of spectrum and the blue-green part are balanced. Thus, when observed under daylight (rich in blue wavelengths), the stone appears blue-green, while under incandescent light (rich in red wavelengths), the stone appears purple-red or brownish-red.

Some of the other stones that show colour change effect include Zultanite (colour-change diaspore), synthetic sapphire with high percentage of vanadium and glasses like Alexite where the colour change effect is due to rare earth elements.

Alexandrite colour change
Alexandrite colour change

References

3 Comments

  • […] The human eye can see about 10 million different colors. It notices these small color changes, adding to the gemstone’s beauty15. […]

  • […] eyes can see about 10 million colors, letting us enjoy the beauty of gemstones2. Elements like Vanadium, Chromium, and Iron change a gemstone’s color. This shows how complex […]

  • […] Learning about light in thin films helps us see gemstones in a new way. By studying their structures and how they work with light, we appreciate their beauty more. Exploring the CIE color specification system helps us understand how gemstones work with light, adding depth to our knowledge of opals and labradorite related discussions on gemstone color15. […]

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