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Gemstone "windows"


#1

Hi everyone, Today I visited a website of a gemstone dealer. A few of
his descriptions used the term “window”. For example: “This gem has
good depth & no window…” and “This gem was faceted a little
shallow, giving it a slight window and making it look like a much
larger one carat gem.”

I just wanted to ask you folks if I understand the term correctly. I
take it that if you compare two gems (of the same type) of equal
carat weight that the one with a shallow cut will display a “clear
area” when viewed from the top whereas the one with a deeper cut
will show more fascets and not have a window. The shallow cut stone
will appear larger when viewed from the top whereas the deeper stone
would appear smaller. Am I correct in assuming that (everything else
being equal) the gemstone with out the window would be more valuable?
Thanks!


#2

Hi Dan,

     This gem has good depth & no window..." and "This gem was
faceted a little shallow, giving it a slight window and making it
look like a much larger one carat gem. 

What’s being said here is that when looking at the stone through the
table, the stone with the ‘window’ acts just like a window, you can
look right through the stone & out the pavilion ( the bottom of the
stone). The other stone has it’s pavilion cut at the proper angle(s)
so the light striking the the pavilion is reflected back to the
viewer. It may not come directly back, but may be bounced from two or
more of the other facets of the stone.

The action of light rays within a transparent stone is governed by
the refractive index of the material the stone is made of. There’s a
’critical angle’ that depends on the refractive index. The lower the
refractive index, the larger (toward 90 deg) the critcal angle is. If
a stones pavilion is cut below this critical angle, no light will be
reflected, you’ll be able to look right through the stone, hence the
term ‘window’.

The formula for determining the critical angle is:

sin c = 1/ri

Where c = critcal angle ri = refractive index

Dave


#3

Dan,

Essentially you are correct. Each stone has a ‘critical angle’,
i.e. an angle below which no main facet on the pavillion should be
cut. For example, quartz has a critical angle of 41deg. There are
two types of facets on the pavillion of a round brilliant…the 16
break facets (those nearest the girdle) and 8 mains (those that meet
at the culet ((point)). If the mains are cut at or below 41 deg, the
path of light entering from the table (top) of the stone will not be
reflected in the stone in such a way as to cause brilliance or other
light giving qualities. Instead, the stone will appear as a piece of
flat glass and you will be able to see directly through the
stone…to the point that you will even be able to read through it.
Most faceters leave at least one degree clearance. Diamonds are
even more critical and huge debates have raged for years about the
proper angles to be used in their cutting. If you really want more
on that, go to www.acagemlab.com

You are correct in assuming that a stone with no window is the more
valuable (all other aspects being equal) because it displays better
light properties and appears brighter. Of course, in colored stones,
depth of color and other factors are important. For example, dark
tourmalines are often cut in such a manner as to cause more light to
pass into and out of the stone simply to enhance the
color…tourmalines are certainly not known for their brillance
anyway. There are also extensive computer programs (cad/cam) that
aide faceters in designing stones and determining the best angles to
achieve the best light qualities. As you know, expecially when
purchasing expensive stones, cut is one of the chief qualities that
must be evaluated. Though presented in somewhat simple terms, hope
this helps.

Cheers from Don at The Charles Belle Studio in SOFL where the white
bellied snow birds will soon be arriving and where simple elegance IS
fine jewelry! @coralnut1


#4

Hi,

I’ll try to answer that. When a (clear) gem is faceted, in general
you are aiming for a maximum return of light, maximum “sparkle”. In
other words, all the light that enters through the top should be
totally reflected back out of the stone by the pavilion facets.

The facet angles which give this total internal reflection depend
upon the index of refraction of the particular gem material. Total
internal reflection also means that you can see into, but not
through, the stone when you look down at it from the top.

If the pavilion facet angles are cut too shallow, you can see
through the stone, and this area is then the “window”.

I recently cut an emerald (emerald cut, in a kind of stubby, coffin
shape) for a friend which unfortunately had a pit at the very bottom
of the pavilion. So I needed to lower the pavilion facets angles in
order to cut this away. The result was a noticeable, albeit small,
cross-shaped window at the bottom of the stone.

Cheers, Hans Durstling Moncton, Canada


#5

Howdy Dan,

I’m sure others hear will respond too.

Almost all cut stones will window if tilted enough. The problem of
course occurs when, in a mostly face up position, there is no
refracted light coming from the main facet area under the table. If a
gem has it’s main pavilion facets cut ‘below’ (numerically less than)
its critical angle (determined by the refractive index) then light
can only reach your eye from that area if it enters the back of the
stone.(ie. “Wow! I can see your finger through your ring’s stone!”)
If the pavilion mains are cut above (180-CA)/3 then you may have
what some folks call a belly cut (there is some disagreement on this
term?) and light can only exit the table after entering the side of
the pavilion and reflecting off one main facet to then shoot up and
out. The goal is to create a stone that has as much of the incident
light falling on the crown returned back out to the observer’s eye.
There are of course compromises and complications. head shadowing,
color saturation, pattern/distribution of scintillation/reflections,
certain odd feature enhancements like ametrine, etc. Also, as RI goes
up this is less of an issue. I’m not sure you’d ever hear of diamond,
CZ or maybe zircon exhibiting windowing. But most other colored
stones could. Most hobby faceters strive to minimize windowing
except possibly if dealing with extremely dark materials (garnets-see
haiku below) where it’s thought to help get apparent brightness up
by allowing windowing. As to value, well that depends on folks
expectations and the design goal of the cutter I guess. Personally,
I’m ashamed of the early stones I cut which window with a very slight
tilt but there is one my wife loves because of the 'twirling dance’
the window does around the pavilion. Go figure.

Choose rough carefully;

Earth’s dark unconquered by man. Garnets yield no light.

Carl
1 Lucky Texan