Australian Facetors' Guild Limited

I am trying to identify the C-axis in some river polished Topaz using a polariscope. Can someone please confirm that my understanding is correct.

Turn polariscope until lens are at 90 degrees.

Place stone on and locate axis where maximum light penetrates the stone.

This is then the C-axis.

Dop stone 7-10 degrees off this axis.

With the polars crossed orient the stone until the stone does not go light and dark, mark this point.  Turn the stone over (i.e. end for end it) and again orient the stone until it does not go light and dark.  A line through the two points is the direction of the C axis.  In other words - A position of single refraction in a doubly refractive stone.  Might be easier to look for a line of colours if there is one - this is the cleavage plane.  Good luck.

John Sellars