A brass olive sized for 5/16 inch tube, used in compression-type tube fittings where a rigid line is clamped and sealed rather than flared. The olive deforms as the nut is tightened, biting onto the tube and forming the seal, which is what makes the joint work. Brass is chosen deliberately: it is softer than the tube it seals against, so it takes the deformation rather than damaging the line. Olives are single-use items. Once a joint has been made and cracked open, fit a fresh olive rather than reusing the flattened original, or you will chase a slow weep that never quite goes away. Keep spares in the toolbox for any hard line work.