Hardcore application in the building process is also as important as any of the key stages . Hardcore is reffered to as some measures of large aggregates of stones used as part of the foundation in a construction project. To simple put is bigger aggregates of what you call granites on site. Though some uses broken blocks /bricks, these are however not recommended because of their strenght, people only use them to save cement during casting of oversite/german concrete which is not advisable.

http://buildingcontractorsecrets.com/2012/05/importance-and-application-of-hardcore-to-a-building/