A nugget is a term for a lump of raw metal. It is common for rocks to contain some percentage of useful metals. But the metallic component of rocks is almost always compounded with some non-metallic elements. Some metals, like Gold and Copper, occur naturally in relatively pure form -- in recognizably metallic form. Pebble or cobble sized natural chunks of metal are called nuggets.
Large gold nuggets are rare enough that they are more valuable as curiosities, than as processed metal.