Question: what are the lines on clay tennis courts made of

title: Clay court
text: Week ATP WTA Week 1 Grand Prix Hassan II ( Casablanca , Morocco ) U.S. Men 's Clay Court Championships ( Houston , United States ) Charleston Open ( Charleston , South Carolina , United States ) Week 2 Monte - Carlo Masters ( Roquebrune - Cap - Martin , France ) Copa Colsanitas ( Bogotá , Colombia ) Week 3 Barcelona Open ( Barcelona , Spain ) Hungarian Open ( Budapest , Hungary ) Women 's Stuttgart Open ( Stuttgart , Germany ) Week 4 Bavarian International Tennis Championships ( Munich , Germany ) Estoril Open ( Estoril , Portugal ) Istanbul Open ( Istanbul , Turkey ) Marrakech Grand Prix ( Marrakesh , Morocco ) Sparta Prague Open ( Prague , Czech Republic ) Week 5 Madrid Open ( Madrid , Spain ) Week 6 Italian Open ( Rome , Italy ) Week 7 Geneva Open ( Geneva , Switzerland ) Open de Nice Côte d'Azur ( Nice , France ) Internationaux de Strasbourg ( Strasbourg , France ) Nuremberg Cup ( Nuremberg , Germany ) Week 8 French Open ( Paris , France ) Week 9