Even as the most infamous cryptocurrencies crashed this year, new ones were already emerging, designed to peg their value to fiat currencies. The most popular of the so-called stablecoins, Tether, broke its peg to the dollar in October, raising questions over the best design for these instruments and the worry that they may be just the latest crypto fad to sucker in the unwary. But if they succeed, stablecoins could prove a tipping point for broader crypto adoption and the reinvention of money.