It is double-lined, meaning it is particularly durable and sewn externally and internally usually from reinforced material such as neoprene which is covered with lining (jersey, polyester, lycra, etc.).
Specifically, the Omer Black Sea is made of double-lined neoprene. Internally, it is made with Open Cells, a very soft and flexible material that does not require lubricants to wear. Externally, it has reinforcements on the chest and knees to offer greater durability and comfort.
The double lining also prevents internal tears.