Typically these 1 jewellers are pin lever movements and the solitary gem is the balance cap jewel on the balance cock. The cap at the other end of the balance staff is a hardened steel plate, and all of the pivots run in plain bearings.
The rationale behind the use of a jewel in this particular location is that most of the time that the watch is not on the wrist it's likely to be sitting dial up (on the bedside table etc), with the balance staff vertical and spinning on the jewelled cap, with wear concentrated at that point. With the watch on the wrist its orientation is constantly changing and so wear is more evenly distributed. If steel were used for this bearing then the staff could effectively drill its way through.
That being said, Timex do seem to have solved the problem with their zero jewel "V-conic" escapements which just keep going with minimal, if indeed any, maintenance.