Place the mastic crystals in a food processor and pulse to get a fine powder.
Put the rice in a large pot. Simmer over medium heat with water and salt for 10'. Add the milk, the sugar and the mastic powder and stir until the sugar dissolves.
when the rice is boiled , dissolve the cornflour in some water and put it into the pot stirring constantly. Continue cooking until the pudding is thick and the rice is tender.
Pour the mixture in 6 bowls or 6 cups, sprinkle with cinnamon and serve hot or cold.