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.
... Jarad-thorn was not in bloom; and the same was the case with the hyacinth (Dipcadi erythraeum), so abundant in the Hisma', which some of us mistook for a "wild onion." The Zayti (Lavandula) had just donned its pretty azure bloom. There were Reseda, wild indigo, Tribulus (terrestris), the blue Aristida, the pale Stipa, and the Bromus grass, red and yellow. The Ratam (spartium), with delicate white and pink blossoms, was a reminiscence of Tenerife and its glorious crater; whilst a little higher up, the amene Cytisus, flowering ... — The Land of Midian, Vol. 1 • Richard Burton
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