Écu

The Écu's silver content was often higher than its face value, leading enterprising souls to melt them down for a quick profit during economic turmoil.

That made it a favorite for smugglers and silversmiths alike in 17th-century France. Minted from .917 fine silver, the coin's weight hovered around 27 grams for most issues, though it varied slightly by reign and mint. Six main mints produced them, from Paris to Bordeaux, each marked with a letter code that numismatists still hunt for today.

Over its 153-year run, the design shifted with every monarch, from Louis XIII's armored bust to Louis XVI's more refined profile. Not all years saw production; records from the 18th century are spotty, with some lost in bureaucratic fires. As for myths about hidden treasures, I've seen enough fakes to know they're mostly wishful thinking.

Some collectors swear by the 1640 issue as the most elegant, but that's just opinion. A dry note: Spotting a genuine one in a dealer's tray is like finding a straight edge on an old guilder—rarer than you'd think.