ring stone

American  

noun

1. a voussoir appearing on a face of an arch.

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Example Sentences

A true topaz crystal was too hard for the ring stone, whose edge slipped over the smooth topaz surface.

From A Text-Book of Precious Stones for Jewelers and the Gem-Loving Public by Wade, Frank Bertram

The scarabaeoid in its turn is abandoned for the simple ring stone.

From Encyclopaedia Britannica, 11th Edition, Volume 11, Slice 5 "Gassendi, Pierre" to "Geocentric" by Various

Gazing into a ball of crystal or glass, a crystal or other smooth ring stone, such as a sapphire or ruby, or even into a common ink-pot, they will see visions very brilliant. 

From The Book of Dreams and Ghosts by Lang, Andrew

The matter being thus introduced, the lady was at once informed that even a real emerald might show signs of wear after a few years of the hard use that comes to a ring stone.

From A Text-Book of Precious Stones for Jewelers and the Gem-Loving Public by Wade, Frank Bertram