by 
The Curious Bag
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A ruby is a red-colored corundum (a corundum with chrome inclusions). A cabochon ruby is a ruby that has been shaped to have a convex top instead of being faceted.
Cabochon
To shape a ruby into a cabochon the gem is cut into a dome and then polished. The back of the cabochon is either concave (curved inwards) or flat. When a gem is cut with a concave back it lightens the color of the gem.
Why Cabochon
Reasons for cutting a gem into a cabochon range from a gem being soft, as an aesthetic preference or because the gem is of lower quality.
Etymology
The word cabochon comes from the Old North French word "caboche" which means "head." When a gem is shaped into a cabochon it is said to have been cut "en cabochon."
Ruby
A ruby is cut en cabochon when it is opaque or when it has rutile (a form of titanium dioxide) inclusions. A ruby with rutile inclusions can be cut to display asterism (a star shape), and is called a star ruby, (a rare form of ruby).
Considerations
Ruby is one of the oldest known gemstones and cabochon is the oldest known cut for gemstones. Until the 1400s, technological limitations kept the cabochon cut as one of two main gemstone cuts, the other being non-symmetrical facets.
Source:
GemSelect: Why Are Some Gems Cut in Facets and Others as Cabochons
Encyclopedia Britannica: Cabochon Cut
Resource:
Lapidary History: Early Gemstone Cuts
AJSGems.com: Cabochon Ruby
Gemstone.org: Ruby
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