Every year since 1866, the nickel has been 75% copper and 25% nickel, except for 4 years during World War II when nickel was needed for the war. Due to the penny's low value, some efforts have been made to eliminate the penny as circulating coinage. Discontinued and canceled coin denominations include:   Collector coins for which everyday transactions are non-existent. Technically, all these coins are still legal tender at face value, though some are far more valuable today for their numismatic value, and for gold and silver coins, their precious metal value. From 1965 to 1970 the Kennedy half dollar was the only "circulating" coin with any silver content, which was removed in 1971 and replaced with cupronickel. However, since 1992, the U.S. Mint has produced special Silver Proof Sets in addition to the regular yearly proof sets with silver dimes, quarters, and half dollars in place of the standard copper-nickel versions. In addition, an experimental $4.00 (Stella) coin was also minted in 1879, but never placed into circulation, and is properly considered to be a pattern rather than an actual coin denomination. The $50 coin mentioned was only produced in 1915 for the Panama-Pacific International Exposition (1915) celebrating the opening of the Panama Canal. Only 1,128 were made, 645 of which were octagonal; this remains the only U.S. coin that was not round as well as the largest and heaviest U.S. coin ever produced. A $100 gold coin was produced in High relief during 2015, although it was primarily produced for collectors, not for general circulation. The United States Mint produces Proof Sets specifically for collectors and speculators. "Silver Proofs" tend to be the standard designs but with the dime, quarter, and half dollar containing 90% silver. Starting in 1983 and ending in 1997, the Mint also produced proof sets containing the year's commemorative coins alongside the regular coins. Another type of proof set is the Presidential Dollar Proof Set where the four special $1 coins are minted each year featuring a president. Because of budget constraints and increasing stockpiles of these relatively unpopular coins, the production of new Presidential dollar coins for circulation was suspended on December 13, 2011, by U.S. Treasury Secretary Timothy F. Geithner. Presidential dollars (along with all other dollar coin series) minted from 2012 onward were made solely for collectors. The first United States dollar was minted in 1794.