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Colored Stones


#1

All,

I was just wondering about colored stones. Perhaps this topic has
come up before.

There seems to be some colored stones out there where color is
paramount. Obviously this important with diamonds and such. I know
the good labs are equipped with spectrophotometers and colorimeters.
L represent lightness level (0-100 units), a is red-green (+/- 50
units), and b is yellow/blue (+/- 50 units). Why don’t they specify
L,a,b color values for stones instead of guessing? Some stones of
value are totally opaque, like black opal. Why don’t they specify
L,a,b of say 25.00, -3.4, 2.4 to quality as a black, and then have
delta E units like +/- 5 units,10 units, and 10 units to be in
specification? You could have similar specifications for other
materials like chrysoprase or lapis lazuli.

You could even use spectrophotometers to check clarity and
transparency through light transmission. You could check to see if
the color of your chrysoprase ring has faded in sunlight, and by how
much.

I mean we all spend up into the $thousands or more for these items.
Why not remove alot of the guesswork?

Blaine Buckman


#2
You could even use spectrophotometers to check clarity and
transparency through light transmission.  You could check to see
if the color of your chrysoprase ring has faded in sunlight, and by
how much. 

Hi Blaine, I completely agree with your question, which has been
mine for a long time. Spectros are commonly used in biological
laboratories for testings, countings, … why not in gemmology ?

Unfortunatly, ALL that I read about that topic claim it’s hopeless
… Why? There are too much parameters, not only a plain color:
quality of the cut and polish, thickness of the stone, brilliance
(internal reflexion), reflexion (external reflexion), inclusions
(just in the very middle of the table? not that easy: close to the
girdle, or the same inclusions close to the culet and seen several
times because of internal reflexions…), color zoning, and so on.
The machine will easily compare a 1ct well cut, top quality stone and
a 5 ct low end one. But its appreciation won’t be the same as yours!

Spectros are very usefull if you have to test hundreds of biological
analysi s a day, because they all come in the same tubes and the
results of the tests are well known, with colorcharts given by the
manufacturers. In gemmology, you won’t have two identical stones: if
you have, it’s calibrated so you don’t need the spectro, because you
know what you have got .

I hope you won’t agree with me, because this topic seems of some
interest to me, and I should like to read different opinions.

Regards,
Yann
www.webcarats.com


#3

Blaine, I think the first problem with dealing with the terms you are
using is that most stones are not sold with lab certificates and
therefore cannot be quantified in the format you are suggesting. The
second problem is that most jewelers don’t have access to the
equipment and therefore cannot reference the color system. The third
problem is that while machines may be able to quantify the amount of
color in a gemstone they cannot quantify the cut, which dramatically
impacts the way the color “works” in a stone. They also are not able
to distinguish the amount of inclusions in a stone. It is not only
the color that makes a colored stone valuable. On top of all of this
everyone has a different perception of beauty. One person’s fine
dark blue sapphire (with whatever numbering system attached to it)
will not necessarily represent another person’s ideal color. Daniel R.
Spirer, GG Spirer Somes Jewelers 1794 Massachusetts Ave Cambridge, MA
02140 617-491-6000 @spirersomes www.spirersomes.com