Generally, the current price (spot price) for gold is based on one futures contract for gold. That futures contract theoretically controls 100 troy ounces, or one brick of gold (1,000 oz for silver). Gold or silver futures are standardized, exchange-traded contracts in which the contract buyer agrees to accept delivery of a specific quantity of gold from the seller at a predetermined price on a future delivery date. The most significant gold futures exchanges are the COMEX in New York City, The Chicago Mercantile Exchange, and the Tokyo Commodity Exchange.
Think of the spot price of gold & silver as a guide correlated with the physical product, sort of like the way the price of oil is loosely tied to what you actually pay at the pump. Many factors make up the premiums on physical gold, and these factors constantly change. The spot price is just one of them.