Ca and Pa variation
In the previous chapter we described what effects is resulted by Pa variation.
But what will happen if we change Pa and Ca at the same time.
Consider two dimensional Ca-Pa variations of the Round Parameters. These parameters were selected, because Ca and Pa give largest input to the diamond optics.
The diagrams show ASET and Office images. Violet square highlights Tolkowsky proportions. The borders of the GIA EX area are shown in gray.
- — Ca-Pa variation
- — Table 57%
- — Star 50%
- — LGF length 80%
Pa variation with Fixed Ca
We already decreased and increased the Pavilion Angle while other fixed parameters. It caused significant ray path change in diamond. So the optical characteristic of diamond changes too.
For example, we modify Pavilion angle from 40.8 for Tolkowsky to 41.4 degrees with other parameters fixed.
It can be seen that the diamond with Pa 41.4 is very different from Tolkowsky both in the ASET image and Office (note the partial leakage under the table).
Pa variation compensation using Ca
But deeper Pavilion can be compensated by Crown angle decrease.
In this case, we can reduce Ca from 34.5 to 31.5 to compensate deep Pavilion.
The resulting proportions of Ca 31.5 Pa 41.4 will be optically close to Tolkowsky, but will have different Ca-Pa parameters.
The combination of Ca-Pa proportions, which give a round diamond optics similar to the reference (Tolkowsky) is called Sweet line.
Diamond proportions positioned on Sweet line can be produced to increase yield and at the same time maintain optical performance.
Anti Sweet Line
The reverse line to Sweet line is called Anti Sweet (Bitter) line. Movement along it leads to a significant change in the optics of the diamond.
Let’s consider the results of vertical deviation from the reference. For Pa 40.2, we get the Head Obscuration area, in which the arrows of the diamond remain always dark during human observation. This is an example of stones where blue in ASET gives a negative contribution to Brilliance by creating constantly black Dead Zones (Dear Arrows) in stone.
The proportions of Pa 41.4 have a partial leakage under the Crown, which we spoke about earlier. The partial leakage area creates both white and black zones during human observation.
Generally Ca-Pa chart can be divided into 3 main areas:
- — area of good proportions optically close to the reference (Sweet line area)
- — area of Dead Zones associated with Head Obscuration (area above Sweet Line)
- — partial Table Leakage area (area below Sweet Line)