I cropped down the area to go over it better, 2 more signals, red arrow could be diamond.
Public places can be a challenge, certainly always plenty of junk. I don't know how much detector manufacturers have improved id. accuracy, but early computerized models on deeper targets would give a lower reading. I had tested digging low end readings and half the targets were nickels. It would say foil/iron at first, but checking as you dig, the id. reading went higher. Then if iron trash was anywhere close to a silver mercury dime, it would read pulltab.