Affixes

Indigo is a tropical plant from which a blue dye is obtained; its main constituent is a crystalline compound either called indigotin or indigo blue, which also occurs in woad; indirubin, indigo red, is an isomer. Sometimes indo‑ refers to the colour, as in indicolite, an indigo-blue gem variety of tourmaline, or the element indium, called that because there are two characteristic indigo lines in its spectrum.

Indole is a crystalline organic compound with an unpleasant odour, present in coal tar and in faeces, first obtained artificially from indigo blue; indene (indole plus ‑ene¹) is a colourless liquid hydrocarbon, obtained from coal tar and used in making synthetic resins; the indoxyl radical —ONC₈H₆ is derived from indole and is present in indigotin; indoleacetic acid is an acetic acid derivative of indole, especially found in plants.