Diamonds may be forever, but a lot of jewelry doesn’t survive the centuries. Rings and bracelets get broken up for re-use, pearls decay, gold is melted down. That explains the excitement over the Museum of London’s new exhibition of the Cheapside Hoard — a trove of almost 500 gemstones and pieces of jewelry from the 16th and 17th centuries, dug up in 1912 by workmen demolishing a building in Cheapside, a busy commercial thoroughfare in the oldest part of London. They took it to a pawnbroker who — fortunately — offered it to a trustee of the museum. The gems caused a sensation when they were displayed in 1914, but have never been shown as a complete collection until now. The exhibit runs through April 27.