Rubia tinctorum, the common madder or dyer's madder, is a plant species in the genus Rubia.

The plant's roots contain several polyphenolic compounds like 1,3-Dihydroxyanthraquinone (purpuroxanthin), 1,4-Dihydroxyanthraquinone (quinizarin), 1,2,4-Trihydroxyanthraquinone (purpurin) and 1,2-dihydroxyanthraquinone (alizarin). This latter gives its red colour to a textile dye known as Rose madder. It was also used as a colourant, especially for paint, that is referred to as Madder lake. The substance was also derived from another species, Rubia cordifolia.

Purpurin extracted from common madder could replace cobalt in lithium-ion batteries.^[1] Eliminating cobalt would mean eliminating a hazardous material, allow batteries to be produced at room temperature, and lower the cost of recycling batteries. Extracting purpurin from farmed madder is a simple task; alternately, the chemical could be synthesized in a lab.^[2]

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