The Test Laps
I had been using tin for some time. Although I didn’t notice any major rounding of the edges, as some people have claimed, it was possible to see the edge as a line of light under a 20X glass if the illumination was arranged correctly. (This is an old toolmaker’s test to see if a tool has a sharp cutting edge).
I had read reports of ceramic producing “flatter facets and sharper edges,” so I obtained a ceramic lap from a US supplier.
I was also kindly given a Corian lap for a trial. Corian is a rather hard polymer. I believe it’s blended with a mineral filler. (Alumina, I think, but the DuPont site isn’t very informative in this respect).
Both the ceramic and Corian laps were new and half an inch thick. They had satin-smooth surfaces with no blemishes.
The Test Gems
For test samples, I used three small pieces of garnet from Tunduru, Tanzania, about 4 mm, dopped with hard wax. They were ground to a pavilion-like point at a 45º angle at indices 0, 32, 64, and 65. I wanted the last two indices deliberately closely spaced, since I reasoned that it…