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.
... ripple as it would a half month later, stood waving over the black-burned ground which the semicivilized Indians had left the fall before. Flowers dotted it, sometimes white like bits of old ivory on the vast rug of spindrift—the pink verbena, the wild indigo, the larkspur and the wild geranium—all woven into a wondrous spangled carpet. At times also appeared the shy buds of the sweet wild rose, loveliest flower of the prairie. Tall rosinweeds began to thrust up rankly, banks of sunflowers prepared ... — The Covered Wagon • Emerson Hough
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