The obvious "woman" piece of currency would be the Sacajawea dollar coin, which has been circulating since 2000. However, even before that both women and minorities have been on currency. In 1886, '91 and '96 Martha Washington appeared on the $1 silver certificate (the one dollar bill) and in more recent memory in the 1980s and 90s Susan B Anthony was on the dollar coin.

Part of the background of the main character in Michener's The Source includes the following excerpt in which the narrator relates the main character's start in numismatics:

Lydia, the first to start making coins, did so at around 600bc but they did not depicted people, but instead famously a lion. Then in Ancient Greece, 479 b.c. King Gelon of Syracuse issued a coin called a Demareteion with a portrait of the Demarete, it is the coin to first feature a head of a person. Another Greek coin from 440 b.c., the most renowned ancient coin, is the Athena's Owl . A silver tetradrachm with Athena's head on the obverse(front) and her sacred owl on the reverse. Many Greek coins were issued with the heads of their gods and goddess, actual people were rarely used. Even Alexander the Great did not have his image on his own coins(after his death, some coins were made with his image).

The nickel, previously, was three cents, and called a trime. The trime was minted during 1851-1873 in two different compositions. The first was metal was silver, silver being such a valuable and precious made three cents worth a very small amount. People would frequently lose such a small coin. The silver trime's weight was only 0.8 grams and the diameter 14mm. Changes were on the horizon for the coin when congress got and approved a proposal from Joseph Wharton, owner of the majority of nickel mines in the United States. Wharton's proposal called for a larger, thicker trime made of a copper-nickel composition. The new trime was 75% copper and 25% nickel started being produced in 1865. The new copper-nickel trime weighed 1.94 grams and had a diameter of 17.9mm. Later, in 1866, congress then felt comfortable to reintroduce the 5 cent nickel, with the same copper-nickel composition although increasing the diameter to 21.21mm, the weight to 5 grams and thickness to 1.79mm. All of those characteristic are what the current nickel is composed of.