What is the ruling on wearing an 'iqal made of natural silk threads?

If the 'Iqal (head-rope) is made of pure silk, then it is forbidden for men to wear it, due to the Prophet's (peace and blessings be upon him) saying: "Indeed, these two [gold and silk] are forbidden for the males of my Ummah." However, if it is mixed with other materials and the silk content does not reach half, then it is permissible to wear it.