Cause Of Colour and Optical Effects In Gemstones

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.

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.

A common example is of Quartz which is silicon dioxide (SiO₂) containing silica ions Si^4+.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.

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.

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.

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.

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.

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

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”.