'Sackcloth' in the Bible

I looked when He (the Lamb) broke open the sixth seal, and there was a great earthquake; and the sun became black as sackcloth [made] of hair, and the whole moon became like blood;

And I will grant authority to My two witnesses, and they will prophesy for twelve hundred and sixty days (forty-two months; three and one-half years), dressed in sackcloth.”