>I wonder if anyone knows how large a spherical crystal ball might be made? >I have >used glass spheres as small as 6" diameter for my sundials but prefer >larger, about 9" diameter. >Is this size a possibility? >Can one machine quartz crystal with tools appropriate for glass? Ie., >diamond saws, drills, etc.
Quartz crystal specimens large enough to make a sphere over a couple inches in diameter, that are free from inclusions, are very scarce. Arkansas quartz might be more affordable than Brazilian, and large clear pieces are available (but VERY few large enough for 6 inch spheres). They are also birefringent, which probably wouldn't matter in a non-magnifying optical system. But you'd be spending many thousands of dollars here, in competition with other users of these spheres that utilize their properties of predicting the future, etc. Quartz is very similar to glass in working properties. Getting even further off-dials: it was noted that a museum in London has a sphere of calcite. These are fascinating objects: you see doubled images, but as you rotate the sphere to look through the optical axis, the images merge. Peter Dollond invented & built an eyepiece micrometer for a telescope using a calibrated lever that rotated such a sphere. The distance between two (stars) in the image was measured by rotating the sphere until the images merged. These spheres are very soft & easily damaged, I scuffed one with a soft pencil eraser. By placing the sphere between crossed polarizers and rotating to view down the optical axis, I saw the most incredibly fine series of diffraction rings, seemingly over ahundred in an inch. Calcite spheres of optical quality are far more expensive than quartz. _______________________________________ Peter Abrahams [EMAIL PROTECTED]
