These are press set (no glue) large, quality fake diamond flesh plugs, in a surgical steel tube. Light can enter from behind. They are held in place with a rubber o-ring which sits in a groove. They are single flared. You might see large jewelled plugs similar to this elsewhere cheaper, but believe me, the quality of these stones is fantastic, they really look stunning.
There is no 12mm size of this particular item as it was was manufactured using American Gauge sizes. The closest to 12mm is 1/2" which is 13mm.
Unless clearly stated otherwise,
the price is for one piece, not a pair.