Tag Archives: gemstone lighting

Gem Cut Grading; Distinctions Without A Difference

The diagram shows the proper angle for viewing an asymetrical gemstone.  The bottom half of the gem is evaluated, the gem is then turned 180 degrees and the other half is judged for percentage of brilliance, the two percentages are then totaled.

The diagram shows the proper angle for viewing an asymetrical gemstone. The bottom half of the gem is evaluated, the gem is then turned 180 degrees and the other half is judged for percentage of brilliance, the two percentages are then totaled.

This matche pair of carat sized blue sapphires exhibit off-axis refraction also known as extinction.  The dark areas move as the gem moves partially defilning the positive areas of scintillation

This matched pair of carat sized blue sapphires exhibit off-axis refraction also known as extinction. The dark negative areas move as the gem moves partially defining the positive areas of sparkle (scintillation) in the gems

I wandered over to one of the major gem forums this morning.   One of the members was talking about a phenomenon he called “shadowing.” This poster defines shadowing when a gemstone exhibits brilliance across half the stone when the stone is held off axis, that is not perpendicular to the light source, he was looking for a cure.

A Non-Issue:

This is a good example of a non-issue.  Non-symmetrical cuts, rectangle, oval, pear viewed under a single beam light source will always show brilliance across half the face of the gem when the stone is tilted away from the perpendicular. Why, because the gemstone is not symmetrical and therefore treats light in a non-symmetrical fashion. In the cuts just mentioned, some facets particularly the pavilion facets which are responsible for delivering brilliance, cannot be uniform.   Some are larger, some are longer therefore light hitting these facets will refract in an irregular fashion.  Symmetrical cuts, rounds and square cushions, by contrast have uniform facet patterns and do not suffer this phenomenon.

For this reason, asymmetrical gems are graded under a single light source, viewed at at a 45 degree angle away from the light source angled toward the eye.  At this viewing angle, half the gem, the bottom half is potentially brilliant.  The percentage of that half is then compared with the other half when the gem is rotated 180 degrees and viewed a second time.  Add the relative percentage of each half and voila you have the total percentage of brilliance.  In colored gemstones, 80% brilliance is considered excellent which means that at any given time, 20% of the gem will exhibit extinction.

One trick for viewing the brilliance of the entire stone at once is the use of sky light.  Turn your back to the sun and view the gem at the same 35 degree angle toward the eye.  In this position the light filtering around the body of the viewer should light up the 100% of the gem.  Any lack of brilliance in this position is a fault and should be subtracted from the theoretical 100% to arrive at the percentage of brilliance.   In most cases, gem photographers correct for this phenomenon by photographing gems with non-symmetrical  outlines by using  multiple light sources.

A Little Extinction Contributes To A Gem’s Beauty

Extinction is another much misunderstood phenomenon. I am often asked, particularly by members of this forum, if a particular gem shows extinction. First lets define our terms, what is extinction. This phenomenon is caused by off-axis refraction. When light enters a gemstone, it enters the crown and reflects internally off the pavilion (back) facets and eventually back through the crown to the eye. Inevitably some light refracts at an angle that is not toward but rather away from the eye, the greater the angle the more extinct it becomes, in a tonal continuum from gray to black.

All faceted gemstones without exception exhibit extinction, no extinction, no scintillation. the great German philosopher Hegel said determinatio est negatio. which when reduced to simplest terms means g all positive requires a negative.  Sweet has no meaning without salty, good does not exist without evil, etcetera.  So too with gems. Scintillation, sparkle is the result of light being broken up into pieces, tiny scintillas of light that are refracted back to the eye in little pieces. Between those pieces is darkness, extinction. Want a gem to light up like a flashlight with no sparkle, eliminate extinction.  Some degree of extinction is therefore required as contrast.

Extinction As A  Fault:

Extinction can have other causes as well, dark is the absence of light.  Gems lacking transparency will often show what one of my clients once called a “heart of darkness” at the center of the gem beneath the table.  This is particularly prevalent in sapphire and the cutter will often cut a window to let in light through the culet, the apex of the cone shaped pavilion of the gem.  So while some extinction is desirable, in fact necessary to the beauty of the gem, large areas of extinction are a definite and definable fault.

Gem Grading: The Death of the Lightbulb and Other Brilliant Ideas

Color temperatures a various types of lighting.  5500-6500 is the Kelvin temperature of daylight.  GIA uses a 6200 Kelvin light source for diamond grading.

Color temperatures a various types of lighting. 5500-6500 is the Kelvin temperature of daylight. GIA uses a 6200 Kelvin light source for diamond grading.

by Richard W. Wise, ©2011

New Technologies May Require Changes is How We Look At Gems:

In 2007 amid little fanfare, Congress passed a law that required that the efficiency of that iconic household standby, the incandescent light bulb, be improved or perhaps accept its doom.  The bulb has been around a long time and the technology has remained virtually unchanged since it was invented by Thomas Edison in 1881.  Turns out the old bulb is a real energy waster, only 10% of the energy used is given off as light, the rest is dissipated as heat.   Though some called it the death of the incandescent light bulb, Congress merely dictated an increase in efficiency, 20% by 2014, 60% by 2020.

Though the efficiency standards do not state what is allowed, such a dramatic increase in efficiency is bound to require new technologies which are likely to mean  changes in the light spectrum produced by whatever technology replaces the old standard.  No one has given much thought to the consequences this will have in the gem trade.  The new standards are scheduled to take effect this January.

In the evaluation of quality, gemstones have been traditionally viewed under two light sources, noon daylight or more lately daylight equivalent fluorescent lighting and plain old incandescent (in the 19th Century it was candlelight).  A stone that looked good by day but muddied up under the lightbulb is taken to be inferior to one that holds its color in both lighting environments.    In 2003 I published a book, Secrets Of The Gem Trade, that divides gems into daystones and nightstones. The terms refer to gem varieties that look best under a  given source.  This seemingly bright idea may mean dramatic reductions in oil imports, but wait!  What about the gem business, what’s a connoisseur to do?

The Tea Party To the Rescue:

Well our worries may be over, just last week the House passed legislation to deny funding to the law.  Apparently the bill’s original Republican sponsor, Texas Representative Michael C. Burgess had an epiphany.  He has seen the, ah, light.  “The government has no right to tell me or any other citizen what type of bulb to use at home,”  no matter how much energy it might save says Mr. Burgess.  We have the right to waste all the energy we like in the privacy of our own bedrooms, says so in the scriptures.

But seriously folks!  Sooner or later, new, more efficient types of lighting are bound to replace the old standby.  Will there be a new standby?  Probably not.  We are pretty much at the point where the type of lighting used will be dictated by the setting that it is used in.  Call it dial a light!  At that point will gem grading light be standardized.  To some degree it already has.  Most laboratories use  some artificial version of daylight.  The Gemological Institute of America (GIA-GTL) uses a 6200 fluorescent bulb, American Gemological Labs uses a 5500 Kelvin bulb.  What is the next step?  Stay tuned to GemWise.

Book Review: The Colour of Paradise, The Emerald In The Age of Gunpowder Empires

Kris Lane

The Colour of Paradise, The Emerald In The Age of Gunpowder Empires

280 pages including appendices

ISBN: 978-0-300-16131-1

9780300161311The history of the gem trade is a difficult research topic because gemstones are very small objects of great value that have been highly sought after for millenia by rich and powerful people looking for wealth that was portable and easily concealed.  The trade itself has been controlled for centuries by minority groups, often oppressed minorities, Jews, Armenians and Indians for whom secrecy was a proven form of self preservation.  Kris Lane is a historian, a Professor of History at the College of William and Mary.  In The Colour of Paradise, Professor Lane focuses very well honed research skills on the history of the emerald, one of the rarest, most mysterious and highly valued of all gemstones.

The book contains no particularly major revelations.  Most historians of the trade are aware that India’s so-called “old mine” emeralds were, in fact, Colombian emeralds imported by the Spanish into India in the Sixteenth and Seventeenth Centuries.   Lane’s contribution is to meticulously document both the early history of Spain’s brutal exploitation of Colombia’s indigenous people and its gem wealth.  He gives us a well documented overview of early trade routes and uncovers some very interesting and original information concerning 16th Century production; methods and emerald values.

Lane begins with the 16th Century and follows emerald production in Colombia right up to the present, with a good account of politics and production into the 1990s.

All and all this well organized and well written account brings real clarity to a relatively murky area of history.  The book also contains detailed appendices estimating early emerald at Muzo, relative values of emerald and diamond in Europe in the 17th Century and  an extensive bibliography.  The author has uncovered several original accounts that have until now been unrecognized.    Highly recommended.