BERLIN — In a city filled with grand monuments to a tragic past, some here say the tiniest are the most powerful: the thousands of brass-topped cobblestones that are spreading over Berlin’s sidewalks.

Six new ones showed up on one street the other day. Five more on another. Each one marks a victim of the Holocaust, placed in front of that person’s home. Together they form an expanding constellation of lives that were lived in this city before it turned against its own.

This elegant capital has a grim, concrete-slab memorial covering five acres next to the Brandenburg Gate that is used for official events to commemorate the 6 million Jews who died in the Holocaust. But now, thousands in Berlin have their own markers, placed there by an artist who has installed similar stones across Europe....