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On pavilion coatings


#1

Hi Folks…

Was reading in a colored diamond article how a supposedly colored
diamond had achieved its fancy color by depositing a colored coating
on its pavilion facets…

So…just for the heck of it, I took a 6mm white round brilliant cut
CZ, and coated the pavilion with red Sharpie marker…

Voila…! Instant pinkish-red CZ…

What I can’t figure out is…since the coating is on the outside
surface of the facets in the case of both stones, how come the light
coming into the stone through the table and upper facets and then
back through the stone changes color…?

I even went into a darkened room, with the stone on a piece of black
cloth and used a penlight…so no light could come in through the
back of the stone…

The CZ is pretty well cut, but I thought, just in case… Same
result…

Does the coating act as the new surface boundary re: reflection on
the pavilion…?

Any ideas…?

Gary W. Bourbonais
A.J.P. (GIA)


#2
     What I can't figure out is....since the coating is on the
*outside* surface of the facets in the case of both stones, how
come the light coming into the stone through the table and upper
facets and then back through the stone changes color....? 

Gary, once you change even the surface color characteristics of a
gem material, you also change its absorption pattern, or the way it
absorbs (and reflects) light, even if in a small way. Take natural
green diamonds, for example. Nearly all natural green diamonds
experienced some radiation after formation, but before being brought
close to the earth’s surface via a kimberlite or lamproite eruption.
As a result, the radiation causes a modification to the diamond’s
crystal structure that is virtually always confined to the surface
"skin" of the rough. These diamonds are usually entrusted to the
finest cutters, who leave as much of that green colored skin intact
at the girdle so that the naturals left behind will contain the
green color that is reflected (hopefully) throughout the diamond.

What happens after irradiation is that the electrons in the carbon
atoms near the surface skin of the diamond behave differently, so
they absorb light differently. A good parallel example is how
amethyst becomes citrine when heated. Most people in the trade
understand that quartz is Silicon Dioxide, or SiO2, and that
amethyst contains trace amounts of iron (Fe) that replaces some of
the Oxygen. This is called a “color center” and it changes the way
the quartz absorbs light. What happens is that neighboring electrons
(called ions) transfer between the Iron and Oxygen atoms, creating
an intervalence charge transfer that causes the quartz to absorb
most colors except the red and blue (mostly yellow, green and
orange) that our eyes view together as purple. Radiation will remove
electrons from the iron impurities, which causes more Oxygen - Iron
intervalence charge transfer, which in turn intensifies the purple
color. Heating the amethyst destroys the color center, which again
changes the absorption pattern to yellow, or citrine. As long as
there are still traces of Iron, irradiation will remove Oxygen from
the Iron atoms and return the purple hue.

So, even if you create a color center only on the surface of a
faceted stone, it’s still a color center and can reflect throughout
a well cut stone but will usually be pink"ish" or green"ish" as
opposed to a nicely saturated gem. This is why people should beware
of intensely vibrant green diamonds…few natural greens are vivid.

In rare cases, such as the Dresden Green diamond, the rough was
exposed to the radiation long enough so that the color is saturated
throughout the stone, but that amount of time is only speculated
upon…nobody really knows…possibly millions of years. This is
often a clue for gemologists to separate natural greens from the
human-irradiated green diamonds I see at gem shows these days. That,
and the extremely low price of irradiated stones. True natural
greens are very rare and very expensive.

    Does the coating act as the new surface boundary re:
reflection on the pavilion...? 

Not so much a new surface boundary as a new “trace element” or
"color center." Obviously, it isn’t absorbed into the stone as a new
element, but it acts like one in that the colored ink from the
sharpie marker is in contact with the material, and so changes the
way it absorbs light. This is the reason gemologists are cautioned
during training to make sure that a stone being color graded doesn’t
touch the masterstone or color sample by which it is being graded.
It’s also why masterstones are usually accompanied by one white- and
one yellow-colored temporary mount for their masterstones. It is to
mount the masterstone in the same colored metal in which a mounted
stone is to be graded. Basically, a stone mounted in yellow gold is
compared to a masterstone that is temporarily mounted in a yellow
mounting to help offset the difference in color the mounted stone
may pick up.

Sorry for being so lengthy but I still left a lot out of the
equation, even so. I’d be happy to muddy the waters further on- or
off-list to anyone.

James S. Duncan, G.G.
James in SoFl