The GRP's core and bottom layer are made with sustainably harvested natural tree rubber. Because the GRP absorbs moisture, it will wear out with use. GRP Series are manufactured through a process that minimizes waste and toxic emissions, while maximizing efficiency in processes and recycling. Crafted in Spain. The GRP Series mat's bottom layer is made of a rubber composite including sustainably harvested natural tree rubber. Because the GRP Series mats absorb moisture, they will wear out with use. GRP Series are manufactured through a process that minimizes waste and toxic emissions, while maximizing efficiency in processes and recycling. Crafted in Spain.