What is a cabochon?

A cabochon is a gemstone that has been shaped and polished. The bottom is flat, and the top is typically shaped into a smooth dome. This is an ancient method of cutting gems and was done before faceting was developed. Though some had carvings on the top (cameos for example). In modern times, a cabochon may be faceted on the top portion of the stone, but the bottom is still flat. 

(I’m providing pictures so this makes more sense)

They are usually round or oval shaped, but I've seen and used other shapes (teardrop and triangle for example).

This technique is also often used for opaque stones. Certain stones are almost always cut "en cabochon", including opal, turquoise, onyx, moonstone, and star sapphire. It’s also common to cut softer stones as cabochons since gems with a hardness score of less than 7 (on the Mohs scale) can easily be scratched during the process of faceting.

Jewelers often use the term "cab" when referring to a cabochon.

Fun fact: The term comes from the French caboche, meaning knob or small dome. 

Want to learn about other types of stone cutting and shapes?
Learn a bit about Polki Diamonds