a head as hard as a stone / stubbornness and inflexibility / an inflexible person

a small stone or pebble, especially one used for throwing

stone

(physics) a neutron

the philosopher's stone

(medicine) doctor

will (intention or decision)

intention; intent; purpose; mind; a wish

a healthy diet and medicine are equally important for health, an apple a day keeps the doctor away

xenotransplantation (transplantation between species) / a xenotransplant