Soak the rice in warm water with a pinch of salt for 20 minutes. Then drain and wash two or three times in cold water until the cloudy water clears.
Put stock, butter and a pinch of salt in a saucepan and bring to the boil
Add the rice and stir once with a wooden spoon. When boiling, add a grain of mastic and turn the heat down and simmer covered.
When the pilav is cooked, remove from the heat,, set aside, covered for 10 minutes, then place it on a warm dish and serve hot.