About 3 million year ago enormous amounts of ashes and rock fragments were erupted from a volcanic rift in the area, where the Goričko region lays today. When this material was depositing and solidifying, layers of basalt tuff and tuffit were formed. These two gray-brown rocks are nicely visible in the quarry in Grad. The main distinctive feature of both rocks is the olivine mineral or the olivine "volcanic bomb" mineral that can measure from one to several centimeters and can be of oval or angular shape. The rock used to be broken to serve as construction material – the Grad castle is also built from it.