Jackleg – A percussion drill used for drifting or stopping that is mounted on a telescopic leg which has an extension of about 2.5 m. The leg and machine are hinged so that the drill need not be in the same direction as the leg.
Jackrock – A caltrop or other object manufactured with one or more rounded or sharpened points, which when placed or thrown present at least one point at such an angle that it is peculiar to and designed for use in puncturing or damaging vehicle tires. Jackrocks are commonly used during labor disputes.
Job Safety Analysis (J.S.A.) – A job breakdown that gives a safe, efficient job procedure.
Joint – A divisional plane or surface that divides a rock and along which there has been no visible movement parallel to the plane or surface.