- Joined
- Feb 26, 2003
- Messages
- 379
The 2008 Journal of Gemmology article "A place for CZ masters in diamond colour
grading", Figure 1 (enclosed) shows the GIA "yardstick' with the D-Z
letter grades and GIA diamond master stone positions which define the
boundaries of these letter grades. Notice that the master stone letter
grade (for example G) defines the upper boundary (F/G) of the range of
a grade (G).
Logic would dictate that any lab grading using this GIA D-Z system and
nomenclature would have to have master stones cloned in some fashion
to match the "master master set" at GIA.
So imagine my surprise and disbelief when I read the IDC rule book
where it defines the masterstone position as being the very bottom of
the grade rather then the top:
3.1.1.4. POSITION OF THE MASTERSTONES
The colour grades are fixed by a series of masterstones, which, as a
definition, fix the lower limit of each colour grade. An international
reference-series has been composed by representatives of the WFDB, the
IDMA and CIBJO.
This position definition is a full grade lower than that of the
originators of the D-Z system of colour grading. Those possessing GIA
diamond masters or equivalent CZ masters would have to relable them
one grade higher (whiter) to accord with the WFDB, IDMA and CIBJO
position definition. In the example the GIA G master (F/G) would be
relabled the IDC F master since it defines the lower boundary of the
range of F.
A stone a half grade whiter than the newly labled IDC F master would
then correctly be labled an F. Using the IDC position rule without
relabling the GIA masters would have you incorrectly calling the stone
a G.
Conversly, using IDC masters with GIA master position definitions,
rather than calling the stone a grade too low would result in calling
a stone a grade too high (too 'white' or colourless).
Is anyone aware of this, or is anyone familiar with why such a
redefinition was instituted? Or can anyone put me in contact with a
representative from one of these organizations who could explain the
rationale for this?
Thanks in advance,
Michael D Cowing
grading", Figure 1 (enclosed) shows the GIA "yardstick' with the D-Z
letter grades and GIA diamond master stone positions which define the
boundaries of these letter grades. Notice that the master stone letter
grade (for example G) defines the upper boundary (F/G) of the range of
a grade (G).
Logic would dictate that any lab grading using this GIA D-Z system and
nomenclature would have to have master stones cloned in some fashion
to match the "master master set" at GIA.
So imagine my surprise and disbelief when I read the IDC rule book
where it defines the masterstone position as being the very bottom of
the grade rather then the top:
3.1.1.4. POSITION OF THE MASTERSTONES
The colour grades are fixed by a series of masterstones, which, as a
definition, fix the lower limit of each colour grade. An international
reference-series has been composed by representatives of the WFDB, the
IDMA and CIBJO.
This position definition is a full grade lower than that of the
originators of the D-Z system of colour grading. Those possessing GIA
diamond masters or equivalent CZ masters would have to relable them
one grade higher (whiter) to accord with the WFDB, IDMA and CIBJO
position definition. In the example the GIA G master (F/G) would be
relabled the IDC F master since it defines the lower boundary of the
range of F.
A stone a half grade whiter than the newly labled IDC F master would
then correctly be labled an F. Using the IDC position rule without
relabling the GIA masters would have you incorrectly calling the stone
a G.
Conversly, using IDC masters with GIA master position definitions,
rather than calling the stone a grade too low would result in calling
a stone a grade too high (too 'white' or colourless).
Is anyone aware of this, or is anyone familiar with why such a
redefinition was instituted? Or can anyone put me in contact with a
representative from one of these organizations who could explain the
rationale for this?
Thanks in advance,
Michael D Cowing
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