Granite and concrete blocks are installed as part of Operational Projects as hardening in order to provide safety for pedestrians. Many granite blocks have been repurposed from bridge rehabilitation projects across the City. These blocks are a sustainable solution that integrates seamlessly into New York City's urban environment. NYC DOT also purchases new blocks when needed.
It's from former mayor Bloomberg's transportation chief Janette Sadik-Khan. He gave her a few million bucks to go ham with experimental street redesigns. She went max-economical and stretched that budget by spending it on street paint, simple street furniture, the adhered sand application you see in the photo (that's not tan paint), and repurposed bollards like those boulders. She grew out the city's class 2 bike lane network with the paint, used the adhered sand and boulders and street furniture to sketch out extra curb space and plazas all over. Many of which were very well-received and got public support to go permanent with actual cement over the following decade. But the city still frequently uses her temporary application approach to test out pedestrianization changes, and the adhered sand / boulder combo works so well it's become pretty much permanent on it's own in some places.