And he knew it, and said, It is my son's coat; an evil beast hath devoured him; Joseph is without doubt rent in pieces. And Jacob rent his clothes, and put sackcloth upon his loins, and mourned for his son many days. Genesis 37:33-34
Rending clothes is a chief sign of mourning in the East. It is customary to lay aside jewels and fine clothing and to be clothed in sackcloth while making loud wailing. See Josh. 7:6; I Sam. 4:12; II Sam. 1:11; 3:31; 13:31; II Ki. 2:12; 18:37; 19:1; Ezra 9:3; Job 1:20.
The ceremony of rending clothes requires the use of a knife. Cuts are first made on the right side of the upper garment for brother, sister, son, daughter, or wife, and on the left side for a father or mother. Then, other garments are likewise cut.
When sackcloth is used it is generally material made of the hair of goats or camels, a cloth that is very coarse and black. Such was used for straining liquids, for sacks, and customarily for mourning garments. Sometimes in mourning sackcloth was worn next to the skin and at other times over the outer garments, or instead of them.
(Dake, Finis Jennings., Dake’s Annotated Reference Bible, Dake Publishing, Inc. Lawrenceville, Georgia, 1999, pg 101)