Durometer is the measure of how hard or soft a gasket material is. Lower durometers are softer and higher durometer are harder. There are several “shores” for measuring durometer.
SHORE A: Most common for most rubbers and foam materials. Common range is 30 Durometer to 90 Durometer. Although softer and harder materials are available. As an unscientific point of reference an eraser is around 30-40 durometer, and a tire tread is 60-70 durometer.
SHORE D: This scale measures hard materials like stiff polyurethane or plastics.
SHORE 00: This scale measures soft materials like sponges and foams.