Cabochons
A cabochon is a gemstone that has generally
been shaped with a flat bottom and a convex top, as opposed to a
faceted gem.
Gemstones that are cut "en cabochon" are
usually opaque gems such as opals.
Faceted gems are normally transparent stones such as diamonds.
This is not a strict rule, however, and exceptions do exist.
Many cabochons are cut with an oval shape to
calibrated dimensions. Some are cut in a freeform shape to
follow the natural patterns of the stone.
Higher quality
Australian opals are very rarely calibrated partly due to
the scarcity of the material. This is especially
true of Lightning Ridge black opals. These opals are
mostly cut in a freeform shape and style to follow the natural
shape and colors of the stone. It is possible to find
lower-quality opals that are calibrated, however.
It is almost impossible to calibrate
fire agates partly due to the
botryoidal, or grape-like, "bubbles" on the surface of the
stone. These rounded shapes hold the "fire", which is a
very thin layer. If a lapidary cuts through this
microscopically thin fire layer the gem will be ruined.
