Indigo
A deep blue dye from the leaves of the indigo plant, used in India for thousands of years.
Also: neel, neeli, avuri
Indigo is made from the leaves of plants of the genus Indigofera. The leaves are steeped and fermented, and the dye is then used in a vat. Cloth comes out of the vat yellow-green and turns blue as it meets the air, and each extra dip gives a darker shade. Indigo is a vat dye: it sticks to cotton and silk without a mordant. It is the blue of Ajrakh, Dabu, kalamkari and many handloom checks and stripes. Today much of the indigo used is synthetic, so a label that says natural indigo is worth asking about.