Gems & Lighting
Manfred Eickhorst, M Sc Physics, Hamburg/Germany
The color impression made by a gemstone is dependent on a remarkable
number of influencing factors. There is the nature of the light source,
the colors of the surroundings and background, and then of course the
ethnic disposition of the man or woman who wears the stone. In addition
to that, the eye of the beholder with its own individual colour
perception and age, as well as its own particular traditional aesthetic
sensibilities all play a part, too.
So it is that this color impression made on the gemstone lover will
vary again and again at different times of the day and in different
seasons, and this too lends a certain charm to our perception of it.
Comparison with Nature’s other coloured “gems” helps us to understand
these interactions; for example the flowers in our garden, whose colour
in the morning may appear to us quite different to their color in the
afternoon or evening. They are a delight to our eyes with the variety
of their color in the natural light of day, but also when used as
decoration in artificial light.
In the flower shop, when they are arranged and presented to us, we are
often astounded by the dramatic beauty of their colors, as we would be
by an actor spotlighted on stage.
No-one would consider lighting that stage as you would an office. What
makes an impression on the observer is good, accentuated light that
brings out the best in its subject.
This applies to the gemstones of the world in a similar way. We don’t
really come to appreciate their beauty until they are captured, like
those actors on stage, in a beam of carefully trained light, such as
LEDs, quartz halogens or full spectrum fluorescents. This in fact is
the secret of the fascination held for diamonds. Its extremely high
refraction means that it lights up even when there is only very little
incident light, though it must be said that the perception of its body
colour is more difficult, and affected by its cut and especially its
size.
Colored stones, on the other hand, have lower refraction, but as they
are typically larger than diamonds, their colour presence leads to a
more intense impression. Having said that, the cut, i.e. the surfaces
by which the incident light is reflected and refracted, is of decisive
importance. It opens our eyes to the colour universe of the gemstones.
The cut is a window to the variety of those colours, which run across
the whole spectrum of the rainbow; colours which, in their origin, lay
hidden within the rough unpolished stone.
The impression made by a coloured gemstone and its sensation to the
observer is influenced by a number of unique color attributes: hue,
lightness (tone/value) and saturation of the colour (chroma/intensity),
as well as dichroism and dispersion.
Together with the influencing factors already referred to, these
determine how we actually perceive the gems and their colors.
On account of these many variables, gemstones have the ability to surprise us in different guises again and again.
Professionals who work in the world of gemstones, such as the members
of the ICA, are by no means immune to this variability either. It is
they who accompany the gem material on their long journey, from the
moment they leave the mine, and get polished, up to the moment they
become parts of jewellery. It is their highly unique abilities which
ensure that the true value of gemstones is brought out and, when all is
said and done, that the wearer can derive pure enjoyment from them. In
view of the diversity already described and the rarity of the stones,
this is thoroughly challenging work. Experience and integrity, and a
background which in many cases goes back over several generations, mean
that the world of coloured gems is quite different to that of diamonds
or pearls.
Thus the right kind of light and the optimum cut supports each stone in
its natural beauty, to the delight of the person who wears it and the
ones who are admiring it.
About the author:
Manfred
Eickhorst received his M Sc from Hamburg Technical College of Applied
Physics. He has developed, designed, and engineered specific lighting
for gems, jewellery, and gemmology. His expertise regarding light and
optics is demanded worldwide. The articles he publishes are well known
contributions as professional references for the gemmological
community.
) 
