What is "natural indigo"

Indigo \In"di*go\, n.; pl. Indigoes. [F. indigo, Sp. indigo, indico, L. indicum indigo, fr. Indicus Indian. See Indian.]

1. A kind of deep blue, one of the seven prismatic colors.

2. (Chem.) A blue dyestuff obtained from several plants belonging to very different genera and orders, such as, the woad, Isatis tinctoria (family Cruciferae), Indigofera suffroticosa, Indigofera tinctoria (family Leguminosae), Indigofera Anil, Nereum tinctorium, Polygonum tinctorium Ait. (family Polygonaceae), etc.; called also natural indigo. It is a dark blue earthy substance, tasteless and odorless, with a copper-violet luster when rubbed. Indigo does not exist in the plants as such, but is obtained by decomposition of the glycoside indican.

   Note: Commercial indigo contains the essential coloring principle indigo blue or indigotine, with several other dyes; as, indigo red, indigo brown, etc., and various impurities. Indigo is insoluble in ordinary reagents, with the exception of strong sulphuric acid.

   Chinese indigo (Bot.), Isatis indigotica, a kind of woad.

   Wild indigo (Bot.), the American herb Baptisia tinctoria which yields a poor quality of indigo, as do several other species of the same genus.