Some useful hand notes to distinguish terminology used to describe the optical effects that cause colour and colour phenomena in gemstones. It’s important to know your spectral colour effects like fire and iridescence and distinguish between pleochroism, iridescence and colour change.
The cover image shows the two body colours (dichroism) that combine in a ruby: orange and pink or purple (credit: Tehmina Goskar).
Visible or white light
Visible light comprises wavelengths of the electromagnetic spectrum from 400-700 nanometers (nm). Violet is the shortest wavelength at around 400nm and red the longest at about 700nm. The spectral colours that make up white light are violet, blue, green, yellow, orange and red.
Body colour
The body colour of a gem material is perceived when some parts of the spectrum of white light are absorbed by it and other parts are reflected back to our eyes resulting in the colour we see.
Body colours are caused by transition elements which are either an essential (idiochromatic) or non-essential (allochromatic) part of the gem’s atomic and chemical structure. Transition elements are also referred to as impurities. Peridot is idiochromatic and coloured by Fe (iron); turquoise is coloured by Cu (copper). Rubies (variety of Corundum) are allochromatic and their colour is caused by Cr (chromium); and so is emerald’s (variety of Beryl) green colour. Blue sapphire, also corundum like ruby, is coloured by Ti (titanium) and Fe (iron).
Massed inclusions can also influence our perception of body colour such as the green of aventurine caused by minute plates of fuchsite, a variety of Mica. Aventurine itself is a variety of microcrystalline Quartz called chalcedony and is colourless.
Colour zoning
Parti-coloured stones such as many kinds of tourmaline exhibit variation in body colour. This should not be confused with pleochroism (see below). This is known as colour zoning and can also be a useful diagnostic tool, for example, colour zoning in a natural sapphire echoing its hexagonal crystal habit or unnatural blobs of colour that don’t seem to go throughout the stone could be signs of dye such as glass paste and dyed quartz.
Dispersion and fire
Dispersion causes the effect known as fire, flashes of spectral colours. Dispersion happens when white light is refracted (bent) into a gem material and then diffracted (split) into its spectral colours, each wave length bending at a different angle with red bending the least and violet the most.
Fire in gemstones is caused by a combination of total internal reflection of light and diffraction in a faceted and transparent gem. In anisotropic or doubly refractive (DR) stones it is also affected by a gem material’s birefringence (the degree to which light splits and bends in anisotropic gemstones). Gems with high relative dispersion are diamond, synthetic moissanite (very high, more than diamond), zircon and demantoid garnet. Also sphene. Cubic Zirconia (CZ) also has high dispersion making it a good diamond simulant.
Interference and iridescence
The rainbow effect. Interference is caused by light being split into its spectral colours through diffraction and reflection off different surfaces in the stone, including inclusions and gem materials made up of very fine layers of minerals, such as feldspars like moonstone and labradorite. A rainbow effect might be seen in thin film inclusions where a thin layer of a liquid is trapped in a fissure and causes iridescence. The orient of peals, that special sheen that is often accompanied by a reflected colour, is also a product of complex iridescence caused by the rough layers of aragonite.
Play of colour
Play of colour is another term for iridescence, different spectral colours viewable when changing the orientation of the stone to reflected light, for example, precious opal’s prized play of colour in black opal, boulder opal, white opal and crystal opal, is caused by the diffraction of light between the minute silicate spheres that make up this gemstone.
Pleochroism
Pleochroism means many colours. It means different colours may be perceived when the material is orientated along its different optical axes. Only anisotropic (DR) stones can be pleochroic and only coloured gemstones can exhibit pleochroism. Garnets are isotropic (singly refractive) and are not pleochroic. There are two forms of pleochroism: dichroism or two colours and trichroism or three colours. You need a dichroscope to detect pleochroism in most coloured gemstones but some have eye visible pleochroism such as iolite (orthorhombic crystal system) which exhibits blue, violet and colourless to brown; tanzanite (also orthorhombic crystal system) which is known for its deep bluish-violet but magenta, pink, golden-brown colours are also seen when moved in different orientations. Heat treatment on tanzanites can disguise its true trichroism. Detecting pleochroism is an important diagnostic tool, such as the purple and orange dichroism of rubies. It is essential knowledge for a lapidary to have so the gem may be cut to get the best colour out of the stone.
Colour change
Do not confuse colour change with pleochroism. The apparent change in the body colour of a gemstone occurs because of different kinds of white light, such as the difference in colour when a stone is viewed in sunlight versus incandescent light. The most famous colour change gemstone is a variety of chrysoberyl called Alexandrite: colloquially described as ’emerald by day, ruby by night’. Colour change garnets are also available. With the increasing sophistication of LED lighting, it is not clear how colour change gems might be marketed.
Source: Gem-A Live: Understanding Gemstone Species and Varieties on Youtube.