Applicable to:
 Brilliant
 Rounded Fancies / Marquise, Oval, Pear
The lower girdle facets are also called Halves.
The lower girdle facet length is usually given relative to
pavilion main facets length. There are several different ways
to measure this parameter giving slightly different results:
 To measure with vertical projection.
 To measure with horizontal projection.
 To measure length differently (nonplane).
To measure with vertical projection
The first way gives the ratio between facets
heights. In the strict sense this is not "facet length" but
this way is most common and unambiguous (figure 1 and figure 2).
Figure 2.
Measurement of depth girdle facets a2 with vertical
projection.
Depth girdle facets a2 is facets heights ratio
on Bezel
Girdle.


Depth girdle facets a1 and a2 differ approximately
on half difference of thickness girdle in points Bone
Girdle and Bezel
Girdle (figure 1 and figure 2). It is difference less
than 0.1 % for a classical symmetric round brilliant.
To measure with horizontal projection
The second way gives the ratio between
facets length, where the lengths are measured in the horizontal
projection.
Length girdle facets b is facets length ratio, b` is facets length ratio along the main facet (figure 3).
Figure 3.
Measurement of length girdle facets b and b`
with horizontal projection.
Length girdle facets is facets length ratio. You may
notice, b` is the same number as depth girdle
facets a2 (figure 2). 

For b= 82,0%, b` = 83,4 %, a1~= a2 = 83,4 %
For b = 75,0%, b` = 76,9 %, a1~= a2 = 76,9 %
Depth girdle facets a2 and a1 is more then Length girdle facets b on 1,4  1,7(%) approximately.
GIA uses b type of measurements. DiamCalc uses a1
and b types of measurements.
Bruce Harding analyse Depth of GIA Pavilion Girdle Facets on the DiamondTalk forum
Measurements are used in different systems and tools
 GIA uses b type of measurements.
 As far as we know: Sarin uses b type of measurement.
 OctoNus Helium uses a1
and b types of measurement.
In Helium system a1 is "depth girdle facets",
b is "length girdle facets".
 OctoNus DiamCalc
uses a1 and b types of measurement.
In DiamCalc software a1 is "lower facets depth",
b is "lower facets length".
Table 1. Names of measurements are used in different systems
and tools.
Type of measurements 
GIA 
Sarin 
Helium (OctoNus) 
DiamCalc (OctoNus) 
a1 


Depth girdle facets 
Lower facets depth 
a2 




b 
Lower girdle length 
Lower girdle facets 
Length girdle facets 
Lower facets length 
Table 2. Values of measurements are used in different
systems and tools.
a) b = 75%
Type of measurements 
GIA 
Sarin 
Helium (OctoNus) 
DiamCalc (OctoNus) 
a1 


76,9% 
76,9% 
b 
75% 
75% 
75% 
75% 
b) b = 80%
Type of measurements 
GIA 
Sarin 
Helium (OctoNus) 
DiamCalc (OctoNus) 
a1 


81,5% 
81,5% 
b 
80% 
80% 
80% 
80% 
c) b = 85%
Type of measurements 
GIA 
Sarin 
Helium (OctoNus) 
DiamCalc (OctoNus) 
a1 


86,1% 
86,1% 
b 
85% 
85% 
85% 
85% 
Comprehensive list of the Helium report parameters:
Measurements are used in different systems and tools:
