Birkenstock Boston Mule The BIRKENSTOCK Boston clog is a veritable classic that can easily be worn all year round. Thanks to its two components, the sole promotes stability and comfort. Cushioning midsole made from EVA and a slip-resistant outsole made from genuine rubber. Fastening:Strap Material Composition:Leather/Leather/Other Materials Anatomically shaped cork-latex footbed. Upper: natural leather. Footbed lining: suede. Sole: slip-resistant rubber-EVA sole. Details: one strap with an individually adjustable metal pin buckle; dual-component sole made from EVA and rubber.