In a large pot, combine the water, sugar, cornstarch solution and gelatin.
Mix well, then stir on medium heat for 30 minute, or until the mixture thickens. Add the lemon juice, rose water, mastic and pistachios. Gently stir until well mixed (about 5 minutes). Remove the mixture from heat and let stand for 10 minutes, or until lukewarm.
Pour mixture in a well oiled 9"x13"x2" (or similar size) pan. Place in the refrigerator until well jelled.
Turn mold out onto a cutting board covered with the corn starch. Sprinkle corn starch on top of the mold so that it doesn't stick. With a lightly oiled knife, cut into desired shapes and sizes. Coat the pieces on all sides with the corn starch.