How the gram price is derived
Although Japanese shops quote per gram, the global market quotes gold in US dollars per troy ounce. One troy ounce equals exactly 31.1034768 grams, so the pure gold (24K) gram price is the ounce price divided by 31.1034768, converted to JPY at the live exchange rate. Lower purities are then a simple multiplication: 22K/916 is the 24K gram price × 0.9167, and 18K is × 0.75.
Japan quotes gold in yen per gram — major houses such as Tanaka Kikinzoku publish a daily retail reference price, and gold futures trade on the Osaka Exchange. The yen price follows the USD spot price converted at the USD/JPY rate, so a weaker yen lifts the local price.