Gold is usually alloyed with some copper as well as other noble metals like silver, platinum and palladium. Gold rarely accumulates into pieces this big. If you find one, its heaviness and distinctive yellow streak serve to identify it. In fact, a streak plate or touchstone has been used for centuries to assess the purity of gold.
Gold nuggets account for very little of the world's gold production. The gold ore being mined today shows no sign of its valuable fraction, yielding a few grams in every ton of rock. Large mines in Nevada, South Africa, Australia, Canada, and the former Soviet Union supply the world with gold for hundreds of industrial uses, bullion, dentistry, coinage, and jewelry. Gold is prized for its electrical conductivity, resistance to corrosion, and extreme malleability. Your computer has gold contacts in it.