Iris or rainbow agate.This translucent form of agate (meaning it lets light through but you can&
Iris or rainbow agate.This translucent form of agate (meaning it lets light through but you can’t read through it) has a special property. When you shine a point of bright light through it, these beautiful spectral colours form. The effect is due to fine bands of inhomogeneities within the agate quite close to the size of the wavelengths of visible light, that vary in refractive index (a measure of how much a substance bends light passing through it). In a manner similar to opal, they act like a diffraction grating, and split light into its constituent spectral colours. The spectral colours are then formed by interference between these split beams of light, and which hue you get depends on the spacing of the banding (and which wavelengths of visible light they approximate to). Only colourless translucent pieces display it, as deep body colour or opacity mask the effect. My gemmological hand spectroscope contains a similar man made grating exploiting the same effect to reveal the absorption pattern of a gem, though some models use the traditional Newtonian glass prism.LozImage credit: Warren Krupsaw.http://www.quartzpage.de/agate.htmlhttp://www.minsocam.org/msa/collectors_corner/arc/iris.htmhttp://www.mindat.org/min-7610.htmlhttp://www.gemdat.org/gem-7610.html -- source link
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