The flight crew stated that the flight was on the final stage of the arrival for landing. The seatbelt sign had been on, the flight attendants had just completed their final pre-landing preparations, and had been instructed to take their seats, when suddenly the flight experienced a sudden jolt due to clear air turbulence. When the turbulence occurred one of the flight attendants was in the process of seating herself and as she placed her hand on her jumpseat to lower it, the turbulence occurred causing her to fall and experience an abrupt and intense pressure on her left hand as her fingers were bent backwards. Her hand immediately began to swell. The injured flight attendant was assisted by another flight attendant, the flight crew was informed, and a medical emergency was declared. After landing the flight attendant was given medical care and diagnosed with a fracture of the 4th metacarpal bone in her left hand.