Spotlight on the gem world: The colour of Rubies.
'The price of wisdom is above rubies' Proverbs 8:11
In last week's spotlight (http://tmblr.co/Zyv2Js1zB9XqS) we mentioned that yellow and blue diamond colours are caused by impurities absorbing some wavelengths of white light, leaving behind the residual colour we see. This week we illustrate the concept with a ruby spectrum as seen through a hand spectroscope. This is an essential (and cheap) part of a field gemmologist's kit. It's basically a tube containing a very fine grating or a calcite prism to separate the white light into its constituent parts. It's also a pretty cool toy, when you look at sunlight (NEVER directly) hundreds of dark lines appear from the elements in the sun absorbing parts of the light it radiates, and when pointed at a fluorescent strip several bright shining fluorescence lines appear. Whenever faced with a strange shapeless waterworn lump of coloured transparent rock, out comes the torch and the tube, shine torch through gem, holding it at end of at end of tube, peer in at other end, and hey presto, spectrum...If they'res one to be seen (not all coloured gems have them).
Practical gemmology requires some manual dexterity. Manipulating a stone held in tongs, with a torch and tube in the other hand, while paying attention to what you're seeing can be pretty fiddly, and just when you're there, ptoing flies the stone out of the tongs. There then follows what's called in the trade 'the jeweller's prayer meeting', everyone gets down on hands and knees and scrutinises the floor intently, moving their heads from side to side, hoping to catch a glint. The gem, following Murphy's law, always grows legs and runs away to the darkest and most obscure corner of the room...
In the case of ruby, you can see that the white light has lost most of its yellow and blue and also has dark lines in the red and blue. These are the absorbed parts of the spectrum leaving the residual red. This is the characteristic pattern of chromium, the impurity causing the colour. The bright red glowing line in the red is the ruby fluorescing, just like the Hope diamond in the last spotlight.
Observant readers will also notice that the emerald spectrum is similar, though with some dark lines in different places. You guessed it, emerald is coloured by chromium too. Now how can one metal cause two different colours? This is due to crystal lattices, these have different structures for each mineral, though all following a set of basic patterns. The chromium is bonded to different places in the two lattice structures, leaving more or less electrons free to absorb the light, and resulting in the varying hues between these gems. Other impurities will modify the colours, for example iron makes rubies brownish or purplish, depending on its electrical state.
The spectroscope allows an informed eye to tell many gems apart instantly by recognising the pattern of lines, including other chromium coloured red gems, such as pyrope garnet and red spinel. However it won't distinguish synthetics since by definition these are identical to real gems, coloured with the same impurities, but man-made.
Image credit: realgems.org
Spectra database: http://www.gemlab.jkharris.co.uk/Spectra%20Database.html
http://chemistry.about.com/od/geochemistry/tp/Gemstone-Colors-And-Transition-Metals.htm
http://www.gemselect.com/other-info/about-gemstone-color.php
http://www.prettyrock.com/spectroscope-instructions.htm
Our piece on glowing diamonds introducing absorption: https://www.facebook.com/photo.php?fbid=492471030813946&set=a.487707811290268.1073741830.352857924775258&type=1
