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Gems and light


G’day; Here we’ve come all over academic again. I have recently
noticed comments about a jewel and it’s light reflecting powers. I
would like to point out that any decent book on gems and gemmology
goes into detail of exactly how light and faceted gems interact, and
it isn’t at all just reflection; it is refraction too.

The object of really good gem faceting is to have any light which
enters the table shoot around inside it by means of a combination of
reflection and refraction and finally emerge via that table and some
of the top facets. Some facets do simply reflect of course but
others reflect it internally, often breaking up the light into
spectral colours when it emerges, thus giving a faceted gem that
delightful fire and sparkle which makes folk want to wave it around to
see the sparkle! Many years ago I used to do a lecture demonstration
using a long narrow glass tank filled with water and a little dye
called fluorescein to allow a ray of light to be seen inside the tank.
(a few drops of red ink works too) I had a slide projector projecting
a thin slit of light parallel with the tank. A small mirror held in a
stand enabled that slit beam to be sent into the water at any angle I
wanted, and a shaken blackboard duster made the beam of light visible.
So; first I had the beam just graze the surface. All the light was
reflected. I twisted the mirror until suddenly you could see the beam
enter the tank, and bend as it struck the water. This is refraction.
Increase the angle further until the bent beam hits the bottom and
shoots up to the surface. Total internal reflection. Then when it
left the surface, it bent once more; refraction again. With care, the
beam could be angled in such a way that the final emergent ray broke
up into a brightly coloured spectrum as it left the water, thus
mimicking what happens in a faceted gemstone. A properly designed gem
should do all of those things. Now, you will see that instead of a
light above your display, you should have lots of tiny, bright,
concealed low wattage lights in reflectors, and your ring and pendant
display will be dazzling! Cheating? I don’t think so; just showing
goods to the best advantage, part of displaypersonship. Yuck; things
get ugly when political correctness is observed! Cheers, – John
Burgess; @John_Burgess2 of Mapua Nelson NZ