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Although various natural materials - particularly henna and indigo - have been used to color hair for at least 3,000 years, several factors contributed to making the 20th century the century of hair color. The development of synthetic organic chemistry occurred in the latter half of the 19th century. Finally, the social acceptance of hair coloring as a fashion accessory led to exponential market growth. Considerable effort has been devoted to trying to simulate the natural hair coloring process whereby the amino-acid tyrosine undergoes several stages of oxidation to produce melanin. Commercial products have not yet been successful, and numerous problems remain to be solved.