What type of socks can be worn as Kuffain?

Assalamualaykum Mufti,
Many Ulama have made permissible Sealskinz socks(http://sealskinz.co.za) for use as Khuffain,Can this be on all hiking/hardwearing socks ect,Do they have to meet certain conditions,Do they have to be waterproof,Are other materials besides leather acceptable
Jazakallah