Hematoxylin is a natural product.
References:
[1]. EF Kingston, et al. Vascular invasion is underrecognized in colorectal cancer using conventional hematoxylin and eosin staining. , 2007, 50(11):1867-1872
[2]. JE Grilley-Olson, et al. Validation of interobserver agreement in lung cancer assessment: hematoxylin-eosin diagnostic reproducibility for non-small cell lung cancer: the 2004 World Health Organization classification and therapeutically relevant subsets. Archives of pathology & laboratory medicine 2013,137(1): 32-40
[3]. HR Zare, et al. Simultaneous determination of ascorbic acid, adrenaline and uric acid at a hematoxylin multi-wall carbon nanotube modified glassy carbon electrode. Sensors & Actuators B Chemical, 2010, 143(2):666-672