The knee cap is actually a bone and when you feel you knee cap you can feel bones because your knee cap is hard.
However the knee cap does contain cartilage to help protect the knee cap bone when it slides during movement.
So you knee cap is both bone and cartilage.
The ends of the femur and the undersides of the patella are covered with a smooth substance called articular cartilage.
The articular cartilage for your knee caps helps the bones glide easily along each other as you move your knees.
Without the articular cartilage protecting your knee cap your knee cap would be damaged from all the movement that your knees and knee caps go through when walking, standing, sitting, running or jogging etc.