From Wikipedia a cabochon is:
A cabochon, from the Middle French caboche (head), is a gemstone which has been shaped and polished as opposed to faceted. The resulting form is usually a convex top with a flat bottom. Cutting en cabochon is usually applied to opaque gems, while faceting is usually applied to transparent stones. Hardness is also taken into account as softer gemstones with a hardness lower than 7 on the Mohs hardness scale are easily scratched, mainly by silicon dioxide in dust and grit. This would quickly make translucent gems unattractive—instead they are polished as cabochons, making the scratches less evident.
A big 'Thank you!" to Bill Tuk for that tip. I've looked at and used cabochons for a while now but I didn't know what these were called. But Bill deserves a bigger "Thank you!" for bringing a bag full of plastic (acrylic?) cabochons to a meeting a while ago. Place the hemispherical cabochon on the XO camera and hold it in place with tape and you have a nice convex magnifier! Of course, Bill tells me (and Mike Lee) that a drop of water works well too. Saliva isn't very clear, so stick with water