What year did pure silver fifty cent pices come out?

The US has never made pure silver half dollars, they have made pure silver Silver Eagle dollars starting in the 1980s, but those coins are intended for bullion value and not for circulation. All US Coins containing silver contain 90% silver at the most because silver is far too weak of a metal to make coins with and needs to be alloyed with a base metal to withstand the wear and tear of circulation.