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Look at the rainbow of colours in soapy bubbles and compare it to the effect of oil spilt on a road on a wet day. When you see this effect on the surface of a gem, you call it a display of ‘iridescence’.
The word, in part, comes from the Greek word “iris” meaning “rainbow” and, in simplistic terms, is caused by multiple reflections from layers within the gem’s surface.Iridescence is caused by diffraction. As white light passes through very small openings such as pores or slits, or through thin layers of material which differ in refractive index, a prism effect causes the light to split into an array of spectral colours. These may then be seen on the surface of a gemstone - or a soap bubble or an oily road.To understand iridescence a little better, turn over a compact disc and gently move it from side to side. Notice how the silver CD starts to display all of the colours of the rainbow. This iridescence is caused by the tiny grooves on the CD which reflect light at different angles from the top to the bottom of the groove.Iridescence is a truly beautiful gemstone phenomenon; we see its effects in the orient of Pearls (where it is known as pearlescence), the displays of Fire Agate, certain Obsidians and Iris Agate. It also creates the beautiful colours of Labradorite and its most well-known example is the colour play of Opals (where it is referred to as opalescence).In Opals the effect is caused not by light refracting at different angles from fine layers as it is with Labradorite or Agate, but from the Opal’s unique inner structure. Unlike the majority of gemstones which have a crystal structure, Opals are created by a mass of small, spherical balls. As light enters an Opal it refracts off these small spheres at different angles. The smaller spheres refract light at a steeper angle, resulting in flashes of blues and greens, whilst the bigger, silica spheres refract light at a smaller angle resulting in flashes of pinks, reds and violets. If an Opal’s iridescence is just one or two colours, then the spheres are therefore all of a similar size, however, infrequently an Opal will be created from a range of different sized spheres and in this rare instance a beautiful kaleidoscopic iridescence may be seen.In Opals, there are several different terms used to describe the style of the gem’s iridescence; Pin fire, Flash, Ribbon and Flagstone Harlequin being the most common ones.Pin fire is a term used to describe an Opal where the array of iridescent colours seen appear like a multitude of pinheads. If you see play of colour and it is formed by lots of individual small dots, rest assured even if you are completely new to gem collecting you would be correct in saying “wow, look at that amazing pin fire play of colour”. Flash colour on the other hand is when the iridescent colours roll across the gemstone in an appearance like a rapidly moving cloud passing over a set of disco lights, each colour seamlessly merging into the next. Those Opals where different colours of iridescence lie next to one another in parallel lines are known as “ribbons”, whilst those with clearly defined colours that sit next to one another like a knitted quilt, with diamond like shapes, squares or blocks are known as “Flagstone Harlequin”.
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