Tim Wescott wrote:

I was going to say "acrylic", but then I realized that maybe I would be
narrowing it down too much.

How hard is it to machine a part out of hard clear plastic and then make
it clear again? Any gotchas? What plastic should one start with? What
question am I failing to ask?

The part will be cylindrically symmetric (i.e., turned on a lathe), about
1.5" diameter, 0.5" tall, and accuracy can be as sloppy as 0.01 in all
directions. But it would need to be "pretty".

Some plastic that is a bit more rugged in impact than acrylic would be
nice (do they make crystal clear Delrin?) but I can't think of anything
like that which would actually work in this case.

Plexiglas can be polished easily after machining. Jeweler's rouge or
similar stages of roughing and finishing compound, then a final pass with
toothpaste to remove any red tint left behind.

If you want pretty and clear, you can't do better than cast acrylic, it
is as clear as glass, maybe even more so.

Jon