The following is one method to measure the crystal's vortex forming area of the eye of the crystal which only occurs in a Vogel-Eredyon type cut crystal. There is also a tool that was developed by one of our colleagues to make this a little easier than the following method with pen and paper.
Step #1 Draw a vertical line on a sheet of paper and align that line with the edge of the crystal's facet in this manner so that the termination angles are perpendicular to the sheet of paper in order to put a flat measuring surface against the crystal's pyramidion
Step #2 Find a thick flat surface straight edge to hold against flat on the crystal's termination angle while keeping the crystal perfectly aligned with the vertical straight line, then draw a line against the straight edge as shown below.
Step #3 Repeat the procedure for the bottom angle while making sure you keep the crystal in alignment with the vertical line.
Step #4 Remove the crystal temporarily and draw a line perpendicular to where these two termination angle lines intersect.
Step #5 Cut
a small piece of plastic tape such as white to see easily in the form
of a diamond or whatever shape to act as a reference point for the area
of the eye of the crystal. Then replace the crystal in the exact position
using the lines on the paper as a position guide. Then place on the crystal
the small piece of tape on line indicating the area of the vortex forming
area of the crystal.
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