small ring gages:

 

the designs for cylindrical ring gages are described in asme standard b47.1.
  rings below 1.510" are manufactured without a flanged outer diameter.
  available in steel, carbide or chrome.

 

large flanged ring gages:

above 1.510" cylindrical rings are manufactured with a flanged diameter as pictured above.
  the removal of excess material from the outside diameter reduces weight for easier use by the operator.
  available in steel, carbide or chrome.

 

jacketed carbide ring gages:

 

pictured above is a "jacketed carbide" ring.
  the insert is solid carbide with the outside jacketed with steel.
  a solid carbide ring would be heavy and subject to damage on the outside due to the carbide being hard and brittle.