Why are Some Gems Cut as Cabochons?

Mixed Gemstone Cabochon Assortment

A cabochon is a stone that is cut with a highly polished rounded or convex top, with or without faceting, with a flat or slightly domed base.

Technically, cabochons are not really "cut." Rather they are shaped and then polished. Before the art of faceting was developed, all gemstones were produced as rounded smooth topped cabochons, though some were intricately carved as well.  It was a much simpler task to produce a cabochon, than it was to cut a faceted gem with many faces.

Faceted gemstones made their appearance in European jewelry during the late 13th and early 14th centuries. Once the technology and skill was available to facet gemstones, why were cabochons still produced? Different gems are cut as cabs for different reasons. In many cases it is because the gem has special properties that are displayed only when it is cut as a cabochon. Examples are gems that display the star effect (asterism) or the cat's eye effect (chatoyancy)  or iridescence (e.g., opal) or adularescence (e.g., moonstone).

Azurite Malachite, Smooth Round Cabs

Most gems that are opaque rather than transparent are cut as cabochons rather than faceted. You will also see lower grade material in gemstones such as sapphire, ruby and garnet cut as cabs. If the gem material has very good color but is not sufficiently transparent or clean to be faceted, it can still be shaped and polished to be a very attractive cabochon. It is also common to cut softer stones as cabs.   Minute scratches show much less on a cabochon than on a faceted stone.

(Information shared from GemSelect.com)