Billon, with its etymological origin from the French bille, is in reference to an alloy of precious metals such as silver and mercury with a majority base metal content (copper, etc.). Billion is used in the making of token coins, coins and medals.
“The use of billon coins dates from ancient Greece through the Middle Ages. During the 6th and 5th centuries BC, some cities on Lesbos Island used coins made of 60% copper and 40% silver. In both ancient times and the Middle Ages, leaner mixtures were adopted, with less than 2% silver content. Billon coins are perhaps best known from the Roman Empire, where progressive debasements of the Roman denarius and the Roman provincial tetradrachm in the 2nd century AD led to declining silver and increasing bronze content in these denominations of coins. Eventually, by the third quarter of the 2nd century AD, these coins were almost entirely bronze, with only a thin coating or even a wash of silver” (Wikipedia).