The Birkenstock Women's Franca Oiled Leather Sandals are a newer, more feminine model. These sandals are stylish and feature crisscrossed straps. The upper is made of oiled leather with an untreated surface. Birkenstock uses sturdy leather with a thickness of 2.8 to 3.2 millimeters as upper material. The Birkenstock Women's Franca Oiled Leather Sandals have an open-pored and thus very breathable surface that makes for a comfortable foot climate, even during more strenuous physical activity.