Kurk (or kork) wool is regarded as the highest quality and best type of wool for its softness and durability; as a result, it is the most expensive wool and costs as much as silk. The best kurk wool also comes from sheep and lambs who live in mountainous cooler regions, who grow full fleeces to stay warm. Kurk wool is very soft because it is taken from young sheep referred to as lambs (sheep under one year in age) and only from the head and shoulder area of the lamb. when they are from seven to eight months old. The very fine strands of kurk wool are woven into a rug very densely to create a very strong, durable and long-lasting rug, and since kurk wool is also very fine, it allows weavers to make very intricate designs and patterns with it. Tabriz, Qum and Isfahan rugs are renowned for their use of the finest kurk wool, which often have silk highlights woven into them.