The Crystal Underground

When Ren Boissevain is not turning motorists' heads in the bumpy-surfaced car he calls his rockmobile or guiding tourists through the cave beneath his shop, he can usually be found down some South American mine shaft in search of rare crystals.

"Crystals are my world," says the former agate miner, showing a visitor through the glittering galleries in the artificial cave—complete with polyurethane stalactites and stalagmites—he's made in the basement of the store.

Called, oddly enough, Crystal Caves, it holds some 600 crystals and fossils, many of which the Dutch-born 69-year-old gathered on...