Frank DeResto, a retired Nassau police officer with the Fallen Officers Association of Nassau County, painted more than 160 rocks last year. The rocks are then placed at the gravesites of first responders who suffered line-of-duty deaths.

DeResto has been painting the stones for about five years. He started by painting the rocks for fallen Nassau and village officers in the county. Now, he's painting stones for first responders from the FDNY, NYPD, Port Authority and Suffolk police.

"We put them on the headstones or at the base of the headstone to mark every grave so that people know, the families know we were there," DeResto told News 12.

DeResto estimates he's painted more than 600 rocks in total.

"This is what we do. We don't want to forget our guys," DeResto said.