One crystal, two colours, no magic… The beauty of gems and crystals is an interaction bet

One crystal, two colours, no magic…The beauty of gems and crystals is an interaction between mind, mineral and light, and some minerals have structures that produce interesting optical effects, opal being an obvious example. The beautiful tanzanite crystal in the photo (see http://on.fb.me/1B8IMQyfor an intro to this purple wonder) is exhibiting another of these properties, which goes under the name of pleochroism, a word coming from the Greek for many colours. A pleochroic crystal actually appears different colours depending on the angle of viewing, and some gems have a very mild version, while others, including tanzanite, a somewhat more extreme one.Crystals are regular arrangements of atoms in a symmetrical repeating lattice, and while the number of individual patterns atoms can order themselves into is almost infinite, all minerals crystallise in one of 6 (or 7 depending how you divide things) crystal systems, based on the length of the axes of their unit cell (the smallest ‘chunk’ of a mineral that can exist) and their angles of intersection. One system is regular and even, known as cubic or isotropic, with equal length axes meeting at right angles, the others exhibit ever greater degrees of wonkiness. This distortion is what allows the gem to play with light in this way and delight our eyes.As light enters the crystal and encounters the lattice, it flows through it down different pathways following the axes, with the rays splitting into two or three. Some directions will be more densely packed with atoms than others, and the light will move at different speeds through them, and be absorbed differently. Since the colour that we see is what is left behind when the crystal has selectively absorbed some of the wavelengths of the light passing through it, different colours can result in the different rays, which then become apparent when the crystal is rotated, or, as in this case, when a change in growth direction occurred.Each pathway through the lattice also polarises the light, forcing it to vibrate in a single direction, and this property is the basis of the gadget gemmologists use to see this phenomenon. Two chunks of Polaroid plastic, orientated at right angles to each other are set in a circle and used to look at the gem. Each half of the Polaroid will show one colour. The property is also useful for distinguishing minerals in thin sections, those slices of rock beloved of geologists.LozImage credit: MIM Museum.https://www.gia.edu/gems-gemology/fall-2014-introduction-pleochroism-faceted-gemshttp://webmineral.com/help/Pleochroism.shtml#.VSvMq5Om0bghttp://www.galleries.com/minerals/property/pleochro.htm7 -- source link

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