"Then Jacob tore (rent-KJV) his clothes, put on sackcloth and morned for his son many days." Gen.37:34
The tearing of clothes is a symbol of inward anguish a mourner is feeling, a voilent expression of emotional pain, and an outward sign to others that the person is suffering great inner turmoil.
In moments of great anguish, the grieving person might tear whatever clothing s/he was wearing and put ashes on his/her head, such as Tamar did after being raped by her brother Amnon: "Tamar put ashes on her head and tore the ornamented robe she was weraing. She put her hand on her head and went away, weeping aloud as she went" (2 Sa.13:19). Her actions represented a widow mourning for her lost husband.
Sackcloth was a rough cloth made of camel's hair, goat hair, hemp, cotton, or flax.
Revelation 6:12 makes reference to sackcloth made of goat's hair: "I watched as he opened the sixth seal. There was a great earthquake. The sun turned black like sackcloth made of goat hair."
Garments made of sackcloth, which was quite often loosely woven like burlap, were worn as a sign of mourning or penitence. These garments were sometimes worn in place of regular clothing. Other times they were simply loose-fitting sacks put over the shoulders, or they could be a loin cloth.
Sometimes the person would put the sackcloth on an ash heap and would lay upon both, instead of wearing the cloth (Isaiah 58:5)
In Nineveh, the command went out that both "man and beast be covered with sackcloth" (Jonah 3:8) as a sign of national repentance.
Other references: 2 Samuel 3:31, 1 Ki.21:27, 2 Ki.19:1, Esther 4:1-2, Job 16:5, and Psalm 30:11
Even today, those descended from Isaac and from Ishmael often tear their clothing to express grief.